A bonus symbol triggers a slot's bonus feature when enough of them land. Unlike a regular symbol it usually pays nothing by itself; its value is the feature it starts.
The trigger rule varies by game. Some need three bonus symbols anywhere in view, which works like a scatter. Others need them on specific reels, for example reels one, three and five, or on an active payline. The paytable spells out the rule.
The feature might be a free spins round, a pick bonus, a wheel bonus or a hold and win round. Landing more bonus symbols than the minimum sometimes gives a bigger version, such as more free spins.
How often the bonus triggers is part of the game's design; see hit rate. On GB-licensed sites you cannot pay to trigger it directly, because bonus buy is not offered.