The Equity is the secure part of the trading account at any given point in time. When an account has open trades the Equity falctuates according to the Live P/L.
The Equity tells you what impact the live trade positions would have on the balance if the positions were closed. For example, if your balance is $10,000 and your Live P/L is + $700, then your equity would be $10,700!
Equity = Balance + Live P/L.