As an Exchange, for a bet to be struck there must be someone else prepared to take the opposing side of the bet. Essentially, both sides have taken each other on. You can get an indication of how much you can wager at a particular price in the market view.
As seen below, the $ number below the odds is the amount that is waiting to be matched by the other side, which is essentially an offer a person (or persons) has placed in the queue. The queue, where bets are waiting to be matched may also be referred to as market depth or volume.
As a result, there are three things that can happen in terms of Matching when having a bet on the Exchange: