Playthrough is the total stake needed to release a bonus, worked out as the bonus amount multiplied by the wagering requirement. A £20 bonus at 10x has a £200 playthrough.
It is the figure that tells you how much play a bonus actually demands, and therefore its cost. Every pound of playthrough is staked at the game's house edge, so the expected loss during clearing is playthrough × (1 − RTP). Illustrative example: £200 at 96% RTP is an expected £8.
Game weighting changes how stakes count. If a game counts at 50%, each £1 staked clears only 50p of playthrough, doubling the real amount you need to bet on it. Some terms also say whether the requirement applies to the bonus alone or to the bonus plus deposit, which can double the figure again; read the terms carefully.