A fully matched bet is when your entire lay stake has been matched at the exchange at the odds that you placed the bet at. 

Your fully matched bet will look different depending on what exchange you're using. 

On Smarkets it would look like this on the right-hand side, you can see the stake is the same as the matched amount (£10 in this example):

On Betfair Exchange, also on the right-hand side under the Open bets tab and here you can see Matched bets:

Finally, on Matchbook again on the right-hand side you can see your matched bets under the Matched tab:

If your bet isn't fully matched please don't worry, it can be fixed. This guide explains about unmatched bets and how to fix them.