A bonus buy lets a player pay a set price, quoted as a multiple of the stake, to skip the base game and enter the bonus round straight away. It is also called a buy feature or feature buy.
Bonus buy is not offered on GB-licensed sites. On 17 January 2020 the Gambling Commission told licensees that bonus buys break its Remote Technical Standards, and since then GB versions of games leave the option out. Versions for other markets may still include it, which is why you may see it in videos or on non-GB sites.
The concern was speed and intensity: a single bought feature can cost many times a normal stake in one go. The bought feature usually has its own RTP, which can differ from the base game's.
For the rules that apply to GB online slots, see UK slot rules and our bonus buy feature page.