A jump-in is the act of jumping towards a grounded opponent while sticking out an air attack.
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.