It isn't an operator of it's own as far as I know. Off the top of my head, there are two uses for it in Javascript (I must assume you are referring to it since you posted in Client Side.) The first is with the ternary operator expression?true-expression:false-expression. The other is involved in JSON where it follows a property/method name
Wait a minute. What's the ternary operator? Is it "?"? I've seen this operator before and it was on my list of things to find out, so to speek. What does this operator to and how does it work?
So if the condition in parenthesis is true then the following after "?" is executed. If false then the following after ":" is executed. In the example above, x equals 5.