It depends on where it's at. Most common use is in a LIKE statement, it is a single character wild card.
ex; LIKE = ???toon will give you all things that has toon with three letters before it such as cartoon or flytoon or bumtoon. See also the * wild card and the %.
I know it appears in the WHERE statement as a variable placeholder that will be filled in, but don't know how that works.
Wait, I just noticed this is in other databases and not mysql.
In which case you're not wrong. MySQL uses % and _, but not all db's do. The OP would need to specify; I know Access for example uses * and ? for theirs. That's an old standard too though, and access 2007 (I think its by default in 89), can be converted to ansi 92 to support % and _.