Start with the if bet. You pick two games and say: if my first bet wins, place my second bet. If the first one loses, the second never goes ahead, so you can only lose one stake.
A reverse bet is two if bets at once, in opposite orders. With picks A and B, you get "if A wins, bet B" and "if B wins, bet A". That covers you whichever game finishes first, and it costs two stakes.
Some books also offer an "action" version, where the second bet goes ahead if the first wins, pushes or is cancelled, not only if it wins.
What a reverse does not do is give you better odds. Each leg is still a normal single at the normal price. It only changes how much you risk and win in each outcome, which the example below shows.
Some people searching "reverse" or "inverse" betting mean something else: betting against a result. On a normal sportsbook you do that by taking the other side (the opposing team, a handicap, or double chance). Be careful with "reverse score" or "anti-score" schemes sold in chat groups as steady income. No bet pays a guaranteed return, and a site that says otherwise is not a sportsbook.