One Time Password also known as One-Time PIN (OTP) is a secret code or password that can be used only once and become invalid after use.
When a user tries to make payment they are sent an OTP via SMS. The user then enters the OTP within a limited time period, typically a few minutes, to validate their identity and complete the login or transaction. If the number is incorrect, the transaction often fails

Example:
For example, if you order a cloth iron from amazon and make payment from your debit card, the bank will send an OTP on your registered mobile number.
Only when you type the OTP will the money be deducted from your bank account and order will be placed