If some combat action causes you to be
knocked from your feet, that is called a 
knockdown.  It ends with you being
on the ground, and an easy target to hit
for a round whilst you get back up.  The round
you spend getting back up, you cannot defend
terribly well.  You can avoid a knockdown with
instant stand or steady footing.  You will
automatically get back up if you are in melee
on your next melee round.