It should be: That depends. We don't say that is depends.
It means it can do it or something can be done but it won't happen unless (and you give what the other person should do).
This is simply speaking. There are many ways to use this phrase, however.
Are you going to the game?
That depends.
On what?
On whether my mother will let me or if she wants me to do something else.
Okay.
Will you help me with my math?
That depends.
On what?
On if you are serious about studying. If you want my help, show me what you know by trying one problem and no playing around while we are studying.
Deal.
