A jump-in is the act of jumping towards a grounded opponent while sticking out an air attack.
Uses
This is primarily used to start or restart offense. Jump-ins usually have lots of frame advantage since you are canceling most of the air attack's recovery by landing.
While a common time to do this is on Okizeme, this isn't required - you just need to get the opponent to stand still long enough for you to jump at them without getting hit. Canceling your Blockstring into a jump-in is a common tactic, as is using an assist for cover.
Deep Jump-in
The amount of frame advantage you get from a jump in varies from move to move, but also varies by how close to the ground you are when you hit the opponent. The closer you are, the sooner you can land and the more frame advantage you get. The trade off is that you are giving the opponent more time to react and counter your your jump-in.
See Also