This is in the Treasure Trap slot machine and is cumulative across all sessions and sittings. Regardless of how much you bet, when 3 or more silver circle Bonus Symbols randomly land in the frame, the Bonus Round will begin. The 1st Room is just a random choice to pretend you have agency. The 2nd, 3rd, and 4th Rooms have 3, 6, and 9 tiles from which to pick, and in each room you must pick a tile with a randomly placed key. Finding a randomly placed skull before the key ends the Bonus Round and gives you a trap towards the lifetime cumulative achievement:
Skulls become more likely than keys, so don't expect a 1 in 8 chance of surviving from the start to the end -- appears to be more like 1 in at least 100. The 5th Room will either be the Treasure Room or a Dead End and will award you with the Safari Jacket. This achievement requires you to eventually get to 5 Dead Ends.
There's no achievement need to bet more than the minimum on this particular game, so it may be wise to save your chips for achievements that do require riskier bets.