Wagering is most commonly associated with casino offers, however, sometimes there will be wagering on sports offers too. 

If an offer states Wager £10 for 50 free spins. The wagering requirement here would be £10, as this is the amount that we need to play through in order to get the free spins. Remember, wagering doesn't mean you have to lose this money - so you wouldn't need to lose £10 in order to get the 50 free spins. 

Wagering may also be listed as a multiplier. E.g. Wager £10 to receive a £10 bonus that must be wagered 10x. In this scenario, the initial £10 is what must be wagered to receive the reward and the bonus funds need to be wagered 10x. It simply means we must wager the £10 bonus ten times (£100) before it will convert into cash funds for us to withdraw.