Auto-spin is a trigger that can be selected within a slot game that will automatically repeat a set spin with a set stake. This essentially allows you to sit back while the system spins the wheel for you, for a pre-set number of games/spins.
This can be selected and de-selected at any time within a slot game. The auto spin will stop automatically if a bonus is hit within the game, or if you have insufficient funds to complete the set number of spins.
You can also chose to set a max loss (most usually within an "advanced settings" option), to ensure you can only spin a certain amount of money before the auto spin function ends. This function will appear differently within each game played, but for example, in one game, it appears as below: