Summary
While in the air, press any of the three Up directions to cancel your current Jump or Super Jump into a Double Jump.
An Extra Boost In Mobility
A few characters have been given the ability to perform a second Jump, known as a Double Jump, after they become airborne. When you perform a Double Jump, you can Double Jump in any of the three directions. This is just another form of extra mobility that your character can possess if you are lucky enough to have a Double Jump and, in Marvel Vs. Capcom 3, any extra mobility is a great benefit.
One of the best features of this is altering the course of your Normal Jump. As mentioned earlier, you do not have much freedom of mobility during a regular Jump, so having a Double Jump allows you to change your course to adapt to the situation. If you Jump at the opponent and they unleash an Assist to lock you down where you land, you can Double Jump backwards to get away from that situation. If you Jump Straight Up and the opponent opens themselves up to an attack, you can Double Jump forward to start the offense.
Double Jumping also works well in Super Jumps to gain even extra height. Sometimes your opponent can fill the ground with a ton of Projectiles that are difficult to clear, and a regular Jump won't get you passed all of the obstacles. Super Jumping can help, but if you can even add that extra Double Jump in air, it can help you get in on the opponent very nicely from the skies.
But perhaps the main added benefit of Double Jumping is the ability to extend your Air Combos. Most every character that possess a Double Jump also has Jump Cancelable Normal Moves in the air so that you can hit the opponent with a bunch of attacks and then Double Jump cancel an attack to connect even more hits, affording you extra damage.
Strategy Corner
Characters with Double Jumps often have cool combos they can perform that have Double Jumps in the middle of the combo. However, some combos allow you to perform Special Moves in the air, continue the combo, and then do a Double Jump later in the combo.
There is an execution issue that has plagued some players in the past where they don't realize they've actually burned their Double Jump by accident when performing a Special Move. For example, Sentinel can actually 
someone in the corner and go up with a Super Jumping Light Attack canceled into a Light Rocket Punch and recover in time to combo another Super Jumping Light Attack into a Medium Attack and then Jump Cancel the Medium Attack and continue the combo from there.
However, most games take into account potentially sloppy execution of the QCF code and figure out ways to make sure you don't see the effect of you accidentally do the QCF motion too far and hitting {{ 
}} by accident at the end of your QCF motion before hitting your attack button. In the case of the combo above, however, if you do that, what will happen is that the Super Jumping Light Attack will actually get canceled into a Double Jump right before the Rocket Punch comes out and, since you are only allowed one Double Jump in the air, you'll be unable to 
the Super Jumping Medium Attack into a Double Jump and it won't be immediately obvious why.
Triple Jumping
Chun Li and Viewtiful Joe actually have the ability to perform a Triple Jump, which allows them to really alter the course of their Jumps for added trickery. What this really means, however, is that Chun Li and Viewtiful Joe are allowed two extra movements in the air. As confusing as this may sound, it's actually better to treat Chun Li and Viewtiful Joe and being able to Double Jump twice in the air.
The reason it's better to think of it this way is because, really, Air Dashes and Double Jumps use up the same resource pool so to speak. If you have both an Air Dash and a Double Jump, you can only choose to do one of these actions while in the air but not both. Characters are typically limited to one movement action in the air, so when you perform an Air Dash or a Double Jump, that one movement action is used up.
However, with Chun Li and Viewtiful Joe, they are allotted two movement actions while in the air! So they can actually perform any combination of their Air Dashes and Double Jumps while in the air limited to only two actions. They can Double Jump twice (thus the Triple Jump), Air Dash twice, Air Dash and then Double Jump, or Double Jump and then Air Dash. So, again, while you may think of Chun Li and Viewtiful Joe being able to Triple Jump, it's more accurate to think of it as being able to Double Jump twice in one jump.