The determination of what our long term expected outcome is, on a given hand, in profit or loss.

On to the example ...

The effective stacks are $1000. Our opponent has a flush draw and we have top pair with top kicker. The pot has $250 and on the flop we bet $175 into it. The opponent shoves over the top for his remaining $875.

What's our EV?

We will win this pot 63% of the time. So our equity is 63%.

The pot now has $250 + $175 + $875 and it's $700 for us to call the shove.

EV = .64 x $1300 - .37 x 700 = $560

On average, we will profit $560 from our call on the flop.