Total balance is the total balance of your account. Available balance is the balance available to you to enter new positions.
For example, if you have a 5BTC total balance and no positions open or pending, you also have 5BTC available balance. If you open a 1BTC margin position, you will have 4BTC available to open more positions (available balance) and 5BTC total balance.
It's pretty straightforward and becomes clear very quickly when you start trading (or demo trading).