This is a defense against a weak two preempt in a major.

A jump shows a game-going hand with 5-5 in the suit and the other major.

Examples:

Opponent You

2H 4C shows clubs and spades

2S 4D shows diamonds and hearts

Other partnerships may play a Roman Jump shows 5-5 in the suit bid and the suit above it. (Thus 4C over 2H would show clubs and diamonds).