For example, the gospel was being preached abroad well before Moses returned the keys of the gathering of Israel. What difference did returning those keys make? It obviously made some important difference, but I am not sure what it is.
Is the meaning that one with keys to preside has the right to direct a matter as he believes right by virtue of his keys if the Lord offers him no clear direction on what needs to be done? Is it that he has the right to determine those matters by right of the keys he holds that are two small to inquire of the Lord over, and which the Holy Ghost does not see fit to clarify?
Or are keys to preside simply the right to instruct those who one is presiding over by revelation? That would make sense, but it leaves open the question about Moses returning keys, for certainly, the gathering of Israel was already presided over by revelation through Joseph the seer before Moses turned keys over to Joseph Smith.
There are certainly multiple kinds of keys. We like to use the word "keys" to mean one exact thing these days, but in the scriptures the word "keys" seems to be used more freely for anything that unlocks something. The same seems to be true of Joseph Smith's statements about keys.
2) There are keys which unlock by providing a way to discern. Such are the three grand keys whereby we can determine whether any ministration is from God given in D&C 129. These are keys in the sense that they unlock our ability to discern whether an angel or spirit that appears is from God or not. These keys do not allow us to preside. In fact, they are keys that are not formed of authority, but of knowledge, for it takes no authority to exercise them or even to convey them to another. They are keys made simply of knowledge.
There is one clear sense in which these do unlock something, and are thereby keys. It is in the sense that doing the full duty associated with the Aaronic priesthood presumably prepares one, in time, to enjoy the ministering of angels. But that hardly seems to warrant the name "keys". Is there more to the meaning?
Yes there is. In the temple we do also learn about what Brigham Young taught.
"Let me give you a definition in brief. Your endowment is, to receive all those ordinances in the house of the Lord, which are necessary for you, after you have departed this life, to enable you to walk back to the presence of the Father, passing the angels who stand as sentinels, being enabled to give them the key words, the signs and tokens, pertaining to the holy Priesthood, and gain your eternal exaltation in spite of earth and hell."
It seems safe to point out that, not surprisingly, some of these key words, signs, and tokens are Aaronic and some are Melchizedek. Of those that are Aaronic some are associated with the ministering of angels and are keys in precisely the sense of the keys in D&C 129. They help us discern whether a ministration is from God.
And while we can't go into it in more detail due to the sacred setting, pondering that relationship makes clear what part of the meaning of the phrase that the Aaronic priesthood holds the keys of the ministering of angels really means.
And once we understand that, then we can, by extension, understand what the intention is when we read:
D&C 84:19 And this greater priesthood administereth the gospel and holdeth the key of the mysteries of the kingdom, even the key of the knowledge of God.