Autoplay is a setting that spins a slot repeatedly without you pressing the button each time, for a chosen number of spins or until a loss or win limit is hit.
It is banned on GB-licensed online slots. The Gambling Commission's game design rules, in force from 31 October 2021, removed autoplay along with turbo spin and slam stops. The concern was that play without a decision on each spin makes it easier to lose track of time and money.
You may still see autoplay in game videos or on versions of games made for other markets. On a GB-licensed site, each spin must be started by you. The rules also require the game to show your net position and how long you have been playing, so you can keep track as you go.