Equity
Equity is your balance adjusted for the open profits and losses of your positions: what your account would be worth if you closed everything.
The balance only changes when trades are closed and when you deposit or withdraw. Equity, on the other hand, moves with every tick while positions are open.
Equity is what drives margin level, free margin and stop out. It is also what you should watch to measure your true drawdown: a steady balance can hide a heavily losing position left open.
The equity curve, which plots this value over time, is one of the best ways to judge a trader. A steady saw-tooth curve is very different from one made of long plateaus followed by sudden drops.