"Free bet" is an umbrella term that covers several different offer types. Classic free bets give you bet credits after a qualifying wager. Matched offers credit you with tokens equal to your first bet. No-deposit-style token offers are rarer and usually smaller.
The key terms to check on any offer: minimum odds for the qualifying bet, expiry window for your credits, eligible markets, and whether the stake is included in any returns. These details are always in the bookmaker's full terms and conditions.
Our golden rule: never claim an offer you don't understand. If the terms are unclear, choose a different bookmaker — transparency is one of the things we score heavily in our reviews.