A phrase referring to the movie Titanic starring Leonardo DiCaprio and Kate Winslet. When Rose (Winslet) is getting ready to go into the water as the Titanic is sinking, she famously exclaims to Jack (DiCaprio), "You jump, I jump, Jack!"
It is a saying that implies that you have faith or trust in someone if they are willing to do it with you.
ex1) P#1: I know that this food looks really gross but it tastes really good, try it.
P#2: Hey, you jump, I jump, Jack
ex2) P#1: This water looks really cold and dangerous.
P#2: Don't worry, I'll be with you the whole time.
P#1: You jump, I jump, Jack!