From scriptures gone over previously we know:
(D&C 13) That the Aaronic priesthood has the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins.
(D&C 28) Joseph Smith held the keys of the mysteries of the kingdom and of the revelations that are sealed until another was appointed his his stead. Consequently other members could speak and teach, but not write, new commandments to the church. They could also teach anything that the Holy Ghost prompted them to concerning the existing revelations and commandments. Joseph Smith was the head of the church and so none in the church was to command him. He was at the head of the church because he held the keys of the mysteries and of the revelations which were sealed. None would be appointed in Joseph Smith's stead to hold the keys of the mysteries and revelations which are sealed except it be done in proper order and by common consent and by the prayer of faith.
In D&C 35 we read
D&C 35:17 And I have sent forth the fulness of my gospel by the hand of my servant Joseph; and in weakness have I blessed him;
18 And I have given unto him the keys of the mysteries of those things which have been sealed, even things which were from the foundation of the world, and the things which shall come from this time until the time of my coming, if he abide in me, and if not, another will I plant in his stead.
25 And Israel shall be saved in mine own due time; and by the keys which I have given shall they be led, and no more confounded at all.
So again, as in D&C 28, we see that Joseph Smith holds the "keys of the mysteries of the mysteries and of the revelations which are sealed", though here it is worded a bit differently and we are told more about it. He held "the keys of the mysteries of those things which have been sealed, even things which were from the foundation of the world, and the things which shall come from this time until the time of my coming". Thus the mysteries and revelations which were sealed are not new, they are old. They were had by Adam, for they were had in the beginning.
And, just as in D&C 28, holding the keys of the mysteries and the revelations which are sealed is paired with leading the church. For verse 25 says "by the keys which I have given shall they [Israel] be led".