A cap on free spin winnings limits how much the spins can turn into, however well they land. Anything above the cap is removed when the bonus converts to cash. Caps are common on spins given without a deposit and can be low enough to make a large hit on the eligible slot pointless. The figure is part of the offer terms and matters most on volatile games, where the rare big result is the whole point of playing.
Winnings from free spins usually arrive as bonus funds with their own wagering requirement, often counted on the winnings alone. Some offers pay spin winnings as cash with no wagering at all, and those offers tend to give fewer spins. A requirement on small winnings can still take a long session to clear, and the balance may run out first. The terms state which of the two applies, usually in the line that describes the spins themselves.
Free spins expire, often within a day or a few days of being credited, and unused spins simply disappear. Larger offers are frequently released in daily batches, each needing a login or a fresh opt in, so missing a day can mean missing a batch. The winnings from the spins then have their own deadline for wagering. Noting both dates when the spins arrive keeps an offer from lapsing unnoticed in the middle of a busy few days.