It simply means someone who thinks they know everything. If you meet a person who always corrects what you say, who tells you trivial facts, who is arrogant about their knowledge, then you can call them a know it all. It can be both a negative or a positive description. The positive way is to use it to describe a friend of yours who is very smart.

adj. <口>假装或自称无所不知的 n. <口>假装或自称无所不知的人