A hold lets you keep one or more reels fixed for the next spin while the rest spin. If two cherries are on the line, holding them gives a chance of a third on the reel that spins.
Fruit machines offer holds at certain times, not on every spin, and the machine decides when. A hold can feel like an obvious opportunity, but the machine's design already accounts for holds in its overall return, so holding well does not let you beat it over time.
Holds are a feature of the British fruit machine along with nudges. The video-slot equivalent is the respin, where the game rather than the player decides what is held. See our nudges and holds feature page.