Wagering is often a term associated with casino offers, but this can also relate to sports offers.
Simply put, if an offer has 2x wagering, for example, on the given bonus/free bet, then we would need to wager/bet the value of the bonus through twice.
We can use Unibet's Welcome Offer for this example.
If your bet loses you will be given a £40 bonus with 3x wagering. This means that you need to wager £120, before either you convert the bonus to cash or the bets lose to the exchange.
You can find the full video for the welcome offer here: