A return is what a spin, bet or session pays back to you. On a slot, the win shown after a spin is the return for that spin, and comparing it with your total bet tells you whether you actually came out ahead.
The distinction matters because a return below your stake is still a loss. A £1 spin that returns £0.40 shows a "win" of £0.40 but leaves you £0.60 down. These are losses disguised as wins, and under the Gambling Commission's game design rules online slots may not celebrate them with win sounds or imagery.
Across all players and many spins, total returns divided by total stakes gives the RTP. For your own play, the useful figure is your net position: what you have got back minus what you have put in. GB-licensed online slots must display it.