It has several meanings. The most simple one is "human".
It is used in Numbers and others (I didn't search hard) to mean "human" or "humanity".
God is not a man, that he should lie, nor as the son of man, that he should be changed. Hath he said then, and will he not do? hath he spoken, and will he not fulfill?
Search
http://www.drbo.org/ for "son of man", and you'll see it used in this sense many times (when you see it, it is a literal phrase from the Hebrew).
It also can be used, based on its usage, to reflect the humanity and humility of Jesus and a perfected Man. So, to say "son of man" is just to say "mere humanity", but when it is used applied to Jesus and in Daniel, it seems to mean "the perfected Man".
It is a common phrase in semitic languages, so when you see it, first check the context.