Availability is how much money is in the markets at the Exchange, it can also be referred to as liquidity.

The amount of money available is shown in £s underneath the market that we're looking to place our bet on. 

Smarkets:

From the image below, using the lay market (blue box) the number underneath is £1,550, this is the availability of the market currently at the price of 2.52.

Betfair Exchange:

Betfair is very similar to Smarkets, but here to place a lay bet you need to use the pink box. Again you can see the availability under the odds. 

Matchbook:

Matchbook is almost identical to Betfair, here we used the pink box to place a lay bet again. 

When laying a bet, we need to make sure that our lay stake is lower than the availability in order for it to fully match. If there isn't enough money in the market then the bet will be either unmatched or partially matched. If a bet is partially matched or unmatched, we need to fix this lay bet. You can find information on how to fix unmatched or partially matched bets here.