Expected value is the average amount you win or lose per bet if the same bet were repeated endlessly. For a slot, the EV of a spin is the stake multiplied by the RTP minus one, which is always negative.
Illustrative example: a £0.50 spin on a 96% RTP game has an EV of £0.50 × (0.96 − 1) = −£0.02. Over 1,000 spins, £500 is staked and the expected loss is £20. The general rule is expected loss = total staked × (1 − RTP).
EV is not a forecast for one session. Variance means real results land above or below it, sometimes by a lot. What EV does tell you is the price of play: a steady cost that grows with the total you stake, which rises with stake size and the number of spins.
At the GB minimum of 2.5 seconds per spin, an online slot cannot exceed 1,440 spins an hour, so stake × 1,440 is the most that can be staked per hour. The RTP calculator does this sum for you.