Withdrawable balance is the amount you can transfer from your Zerodha account to your primary bank account.
This balance may differ from the net available funds shown in your funds statement because exchanges don't settle funds from equity and F&O trades instantly. Exchanges require T+1 day to settle funds from selling equities and F&O trades to your Zerodha account before you can withdraw them.
The withdrawable balance may change after market hours if you have traded during the day. This happens because Zerodha updates all charges and obligations after the end-of-day process between 5 PM and 9 PM. Pledging stocks can also affect your withdrawable balance.
While you can use funds deposited to your trading account immediately for trading, you can only withdraw them the next day due to end-of-day settlement and reconciliation processes.