Its also called a "fish bet", because thats what it is. A very fishy move, and as others have said its usually a weak hand. The way to approach the situation is to pretend, that they checked to you and then do, what you would do against a check. If you would C-bet, then you put in a raise, and if you would have checked back, you just call the silly fish bet. One thing to be aware though, is that a fish bet followed by a 3-bet is actually a very strong line. Its essentially the same as a check-raise but generally skewed more towards value.