Drawdown is the decline from a peak in your account value to the subsequent trough, usually expressed as a percentage. If your account hits $10,000 and falls to $8,000 before recovering, that is a 20% drawdown. It measures the pain along the way - not where you end up, but how deep the hole got.
The recovery math is brutal
Here is the part that surprises people: losses and the gains needed to recover them are not symmetric. A 20% loss requires a 25% gain to break even. A 50% loss requires a 100% gain. A 90% loss requires a 900% gain. The deeper the drawdown, the exponentially harder the climb back - which is why capital preservation is not cautious, it is mathematical.
Why it matters more than return
A strategy's headline return is meaningless if the drawdown to get there would have made you quit - or blown the account before the recovery arrived. Two systems can post the same annual return; the one with the shallower drawdown is dramatically easier to actually stick with. Survivability, not peak return, is what compounds.
You cannot compound an account you have already blown up. Protecting the downside is the whole game.
Controlling it
Drawdown is managed with the unglamorous tools: sane position sizing, a defined risk-reward ratio, and hard daily loss limits that stop the bleeding before a bad day becomes a catastrophic one. A daily circuit breaker - a rule that halts trading after a set loss - exists for exactly this reason: to cap the depth of the hole before emotion digs it deeper.