Being knocked down means your hitpoint is reduced to 0 or less.
However if the damage that spills over is more than half your max hitpoints(round up). You are immediately considered to have failed all death saving throws
When you are knocked down, you gain a value called your knocked down HP equal to hit die max + level + constitution modifier.
Each time you take damage less than your Knock down HP, you fail one death saving throw, if the damage is equal or more, you fail two death saving throw.
When you are knocked down you are considered immobalised.
When you are knocked down, at the start of your turn you must make a death saving throw. Roll a D20, if you roll 10 and above you gain a success, if you roll 9 and below you gain a failure.
Death Saving throw success and failures reset on gaining any hitpoints.
At 3 success you become stabalised, and at 3 failures you are killed.
Rolling 1 counts as 2 failures and rolling 20 makes you return back up at 1 Hitpoint.
For each death saving throw you fail gain Wounded 1 stacking. A wounded value is a penalty to all D20 skill checks or attack roll.
You lose <Consitition modifier> of wounded after every long rest.
Wounded can be removed with certain spells or abilities.