Two very different things share the name. Inside a game, free spins are a feature triggered by symbols, already priced into the RTP. As a promotion, they are a marketing offer: fifty spins at a stated stake, whose winnings typically become bonus funds subject to a wagering requirement and often a cap on what can be withdrawn.
A rough valuation is spins multiplied by stake multiplied by the game's RTP: fifty spins at 0.10 on a 96% game are worth about 4.8 before conditions. Wagering requirements and withdrawal caps then reduce that further, which is why an offer of '50 free spins' is generally worth a small fraction of what the number suggests.
Why it matters
Spins are the most advertised bonus format because a count of spins sounds larger than its value. Pricing the offer takes one multiplication, and the terms below the banner decide most of the answer.
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