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. Once the opponent is conditioned to block a jump-in, you can start going for other things, like Empty Jumps.
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.
The amount of frame advantage you get from a jump in varies from move to move, but also varies by how you land after hiting the opponent. The sooner you can land means the sooner you cancel your recovery, and the more frame advantage you get - this is known as a deep jump-in. The trade off is that you are giving the opponent more time to react and counter your jump-in. However if the opponent can not counter your deep jump-in (such as during Okizeme), then there is no risk to doing a deep jump-in for maximum frame advantage.
In games with more air movement options like BlazBlue and Guilty Gear, you can stall your air momentum by air dashing right before you land to give your jump-in a bit more variation. This makes defense more difficult by forcing the opponent to not rely solely on Guard Switch.