A knockdown occurs any time your monster takes 25 or more damage. Since blocking subtracts your monster’s block rating from the damage taken, blocking can often prevent your monster from being knocked down.
When a knockdown occurs, the battle is halted until the monster on the ground gets back up and moves back to its battle start position. Monsters do not regen stamina while knocked down.
As a blocking defender, it’s always better to not get knocked down. Blocking doesn’t only reduce the damage your monster takes, but it also allows your monster to counter, forcing your opponent to become the defender. If you get knocked down during your block though, you cannot counter and your opponent will also gain a stamina lead while your monster is down. This is why having a good block rating and having reliable knockdown abilities can be important.