Given that it dominates Christ's discussions about Himself, particularly in John, it is surprising how little discussion occurs over what Jesus meant by the term "Son of Man."
There have been a few explanations given. A particularly biased answer is that Jesus is making a claim to Messiahship [or even divinity] by using the expression because it is an homage to Daniel 7:13, where the apocalypse is described with "a son of man" riding on clouds.
To claim "son of man" was a title of sorts due to this single reference is rather a reach since just a chapter later in the same book Daniel himself is referred to as a "son of man" [Daniel 8:17].
To suggest more and claim the cloud imagery means that this figure is actually God goes beyond merely "puzzling" for in the verse we are told the "son of man" is presented to God [the Ancient of Days]. (And it also is confounded by Numbers 23:19 that says "God is not a son of man."
Some research has suggested the term "Son of Man" is an aramaic idiom that means "me." This would make it similar to "Yours truly" when used to refer to oneself. The problem with that idea is that the Jews do not seem to understand what Jesus is referring to when He speaks of the "son of man" in John 12:34. Indeed, that verse makes nearly impossible that the expression is just an idiom for "me."
Another option that has been put forward is that it is just a way of saying "a human." That certainly makes sense given the many, many times the expression is used in the OT. However, it once again does not make sense with its usage in John 12:34 or elsewhere.
I think that perhaps Jesus is calling attention to Ezekiel. Throughout Ezekiel, God addresses the prophet as "son of man." The phrase is used there about 80 times, more than five times as often as all the other books of the OT combined. This is illuminating because it means we can see Ezekiel as a Type of Christ and gain an understanding of Christ's work by looking through Ezekiel's actions in the OT.