What is the withdrawable balance?
The withdrawable balance is the amount of money that can be withdrawn to your bank account from your trading account. It may differ from the net available funds shown in the funds statement as the cash from equity and F&O trades are not settled instantly. Cash from selling equities takes T+2 days and from F&O trades take T+1 day to be credited to your trading account by the exchanges before you can withdraw them. See What does settlement cycle i.e rolling settlement mean?
The withdrawable balance can change post market closing if you have traded during the day. This is because all charges and obligations gets updated post the end of the day process typically between 5 PM to 9 PM.
Pledging stocks can affect the withdrawable balance. See
How is withdrawable balance calculated after pledging?