Okizeme (often shortened to oki) is a Japanese fighting game term (起き攻め) that refers to attacking an opponent while they are rising after getting knocked down. Because the opponent is stuck in a wake-up animation after being knocked down, this is a great time to do meaties, safe jumps and mixups. Each character has their own specific oki options, but most can be grouped into a few categories. Each has it's own strengths and weaknesses.
The primary trait all okizeme options share is that the opponent is in a state where they can not retaliate, allowing you to do slower, more rewarding attacks while minimizing risk. In games like Guilty Gear, nearly all characters have strong oki options and players should be attempting to creating as many oki situations a possible - mostly by ending combos with moves that knock down the opponent.