What It Means
Ledger Balance is your actual cash position, carried from your account's running ledger of deposits, withdrawals, and settled realised profit or loss. Margin Available is the broader, real-time trading limit your broker computes for you right now — and it can be a bigger number than your ledger balance.
How It Works
Margin Available folds in things Ledger Balance doesn't: the usable value of pledged shares after haircut, premium received from selling options, and unrealized or not-yet-settled amounts. It tells you what you can trade with today; Ledger Balance tells you what cash is actually sitting in your account.
Warning
Don't treat your full "margin available" figure as withdrawable cash — a large chunk of it can be collateral-based trading limit, not money you can actually pull out to your bank account.