Availability is how much money is in a select market on the Exchange (Pocketbet, Betfair, Matchbook) at any one time.
This is always shown below the market where we are looking to place our lay bet.
Pocketbet
Pocketbet works slightly differently to other exchanges as instead of availability it just shows you the max lay stake you can place at the odds selected. Once you click on a bet, you will see the max stake on the bet slip. In the screenshot below it shows the maximum lay stake you could place is £101.
Betfair Exchange:
Betfair is laid out very much the same, it is just a pink box. You can see that the price is the same as Smarkets, but it has more availability.
Matchbook:
Again, Matchbook looks almost identical to Betfair. It shows the same price but less availability.
We have more information about how lay bets work in the video below: