Wikipedia isn't really a place to share your knowledge—it's a place to share what you have read elsewhere. It's the basis for a policy called "No original research", and it works like this:
If you write that Babe Ruth played for the Red Sox and Yankees, you probably don't need a source, because it isn't likely to be challenged.
If you write that Ruth's contract was sold by the Red Sox to the Yankees on December 26, 1919 that's specific enough that you should include one.
If you write that Harry Frazee sold Ruth to settle a gambling debt because your great-uncle swore it was the truth, it doesn't belong (sorry, Uncle Rog).
The point isn't that you must include a source for everything, but everything must be traceable to a published source. But why? More on that next week.