Quick example since I have a few minutes to spare:

The pot is $15 on the flop with $12 behind and we're facing a single opponent who open shoves. We suspect that we have about 40 percent equity when we call.

The outcomes after we call:

1. We win (happens 40% of the time, we profit $27).
2. We lose (happens 60% of the time, we profit -$12).

EV of calling = (0.40)(27) + (0.60)(-12)
EV of calling = $10.80 - $7.20
EV of calling = $3.60