Okizeme (often shortened to oki) is a Japanese fighting game term (起き攻め) that refers to attacking an opponent while they are knocked down. Because the opponent is stuck in a recovery animation while knocked down, this is a great time to do meaties, safe jumps and mixups. There are various types of oki, and 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 with little risk.