Balance available is (Effectively) your credit limit minus your current balance due and any pending charges. It's the credit equivalent of your bank account (positive) balance: how much could you bill/charge if you wanted to. With a balance available of $500, you could charge something for $480, but you wouldn't be allowed to charge $530.
With a limit of 500, you might have:
Balance (due): $0
Balance Available: $500
Balance (due): $50
Balance Available: $450
etc. The available balance also takes into account various credit holds (like if you go to the gas station, the pay-at-pump puts a $100 hold on for a day or two before you pump, to make sure you can cover the actual cost - it will be adjusted down once the reconciliation occurs, and you'll never be charged interest for it, but it does reduce your balance available. Those holds are often included in "Pending Charges" or similar.
Pending Charges: $75
Balance Available: $425
You can also have other kinds of available balance - like available for cash advance, etc. - that might be lower.