JOEL - There was a large portion of the gold plates(approx 2/3) that were sealed that Joseph Smith was commanded not to to translate.

Speaking of the Book of Mormon God said:
"And he has translated the book, even that part which I have commanded him, and as your Lord and your God liveth it is true." (D&C 17:6)

And in Ether:
"And now I, Moroni, have written the words which were commanded me, according to my memory; and I have told you the things which I have sealed up; therefore touch them not in order that ye may translate; for that thing is forbidden you, except by and by it shall be wisdom in God."(Ether 5:1)

The sealed portion contains a vision the Brother of Jared had about the beginning to the end of the world, similar to what John the Revelator saw, as described in the following verses in Ether:

And when the Lord had said these words, he showed unto the brother of Jared all the inhabitants of the earth which had been, and also all that would be; and he withheld them not from his sight, even unto the ends of the earth.
For he had said unto him in times before, that if he would believe in him that he could show unto him call things—it should be shown unto him; therefore the Lord could not withhold anything from him, for he knew that the Lord could show him all things.
And the Lord said unto him: Write these things and seal them up; and I will show them in mine own due time unto the children of men.
And it came to pass that the Lord commanded him that he should seal up the two stones which he had received, and show them not, until the Lord should show them unto the children of men. (Ether 3:25-28)

All what the Brother of Jared saw will some day be revealed to us as recorded in Chapter 4 of Ether:

And the Lord commanded the brother of Jared to go down out of the amount from the presence of the Lord, and write the things which he had seen; and they were forbidden to come unto the children of men until after that he should be lifted up upon the cross; and for this cause did king Mosiah keep them, that they should not come unto the world until after Christ should show himself unto his people.
Behold, I have written upon these plates the very things which the brother of Jared saw; and there never were greater things made manifest than those which were made manifest unto the brother of Jared.
Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord.
For the Lord said unto me: They shall not go forth unto the Gentiles until the day that they shall repent of their iniquity, and become clean before the Lord.
And in that day that they shall exercise faith in me, saith the Lord, even as the brother of Jared did, that they may become sanctified in me, then will I manifest unto them the things which the brother of Jared saw, even to the unfolding unto them all my revelations, saith Jesus Christ, the Son of God, the Father of the heavens and of the earth, and all things that in them are. (Ether 4:1-7, See also 2 Nephi 27:7-10)

Events surrounding the translation of the gold plates and the sealed portion were prophesied by the prophet Isaiah:

"And the vision of all is become unto you as the words of a book that is sealed, which men deliver to one that is learned, saying, Read this, I pray thee: and he saith, I cannot; for it is sealed:
And the book is delivered to him that is not learned, saying, Read this, I pray thee: and he saith, I am not learned." (Isa. 29: 11-12)

This prophecy of Isaiah was fulfilled when Joseph Smith(representing the one who is not learned) sent an example of the characters from the plates along with the translation to a professor Charles Anthon(representing the one who was learned) to get his opinion about the accuracy of the translation.
Martin Harris who delevered the writings to the professor described the following experience:

“I went to the city of New York, and presented the characters which had been translated, with the translation thereof, to Professor Charles Anthon, a gentleman celebrated for his literary attainments. Professor Anthon stated that the translation was correct, more so than any he had before seen translated from the Egyptian. I then showed him those which were not yet translated, and he said that they were Egyptian, Chaldaic, Assyriac, and Arabic; and he said they were true characters. He gave me a certificate, certifying to the people of Palmyra that they were true characters, and that the translation of such of them as had been translated was also correct. I took the certificate and put it into my pocket, and was just leaving the house, when Mr. Anthon called me back, and asked me how the young man found out that there were gold plates in the place where he found them. I answered that an angel of God had revealed it unto him.
“He then said to me, ‘Let me see that certificate.’ I accordingly took it out of my pocket and gave it to him, when he took it and tore it to pieces, saying that there was no such thing now as ministering of angels, and that if I would bring the plates to him he would translate them. I informed him that part of the plates were sealed, and that I was forbidden to bring them. He replied, ‘I cannot read a sealed book.’ I left him and went to Dr. Mitchell, who sanctioned what Professor Anthon had said respecting both the characters and the translation.” (Joseph Smith History 1:64-65)

Some day(probably during the Millennium), when our hearts are prepared to receive it, the gold plates will be delivered to a prophet of God who will be directed to translate the sealed portion so we may know what the Brother of Jared saw in his vision.