Game weighting sets how much of each stake on a particular game counts towards a bonus's wagering requirement. Slots often count at 100%, while table games and live casino may count at a lower percentage or not at all.
The effect is easy to underestimate. At 100%, a £1 spin clears £1 of playthrough. At 20%, a £1 stake clears only 20p, so you need five times as much play on that game to clear the same bonus, with five times the expected cost.
Casinos use weighting mainly because games with a low house edge would otherwise make bonuses cheap to clear. Some also exclude specific slots, often those with high RTPs or certain jackpot games. The bonus terms list the weightings and any excluded games.