Before getting into sportsbook promotions specifically, it helps to understand hedging on its own, since it is the single underlying concept every matched bet is actually built from.
What hedging means
Hedging means placing a second bet that covers the opposite outcome of a first bet, so that the combined result of both bets is far less sensitive to which side actually wins. In its purest form, a perfectly sized hedge makes the outcome of the event almost irrelevant to your combined result, you come out close to the same place either way.