Knockdown has to do with when a character falls over due to taking an attack. This state is known as being “down”, among other names. The act of getting up from this state is termed “wakeup” in fighting game jargon, though the uninitiated (or developers, foreign speakers, etc.) may use terms like “standing” or “rising”.
Largely, one goes tumbling into this state due to sweeps.
Relief or not, this state is a bit risky, as if anyone is left standing, they will often be able to transition to meaty attacks. The person who has to get up has the riskier side of this equation: wakeup attacks.
In 3D games, you will often be able to retaliate from this state with wakeup kicks.