From Halopedia, the Halo wiki
Ghost Jumping is a very easy-to-learn skill. It is plainly using the Ghost to ride over a ramp to reach a high altitude. You can reach higher altitudes from a Ghost by getting out of the Ghost while in mid-air. Also, by holding the "A" button, you engage the air-brake and raise the nose, making you go higher still.
To maximize this trick, you should boost until you're behind the jump-area. If you continue right until you reach where you want to jump, then you will pass over it, which is usually not advised. Therefore, be careful when using the boost.