The statement is relatively cryptic. It is often assumed to be a shortening of one saying "Go ask that girl out." However the exact girl to be asked out usually is a mystery and the meaning can be interpreted in different ways.
Christopher, Jack and Tim were sitting at a table at school. In the same room were a few other girls. Suddenly Christopher smirks and says to Tim "Go For It." Tim then replies back "Go For It" giving Christopher a taste of his own medicine. Jack meanwhile sits amused watching the two but finally can no longer resist the urge he has and says "Go For It" himself.