Custom nonce is an option that some wallets offer to manually set their nonce value for each transaction they send. You can use this option to send another transaction with the same nonce value but with a higher gas price as your pending transaction. Alternatively, you can also send another transaction with zero value and zero data to yourself with the same nonce value as your pending transaction. This will effectively cancel your pending transaction, as the network will only accept one transaction with the same nonce value from the same sender.