The earned value (EV) is the amount of work completed expressed in monetary terms. It is the amount from the budget for that work.
The PMBOK defines the equation for earned value as "EV = sum of the planned value of completed work". The earned value is used in many other project management calculations (see table 7-1 of PMBOK 5th edition). It is used to calculate variances from actual amounts as the project progresses. Then those variances and the budget can be used to adjust the cost forecasts.