An OTP (One-Time PIN) code is a one-time password or one-time PIN. An OTP is a feature of Two-factor authentication (also known as 2FA) security. When a user attempts to access a service protected by two-factor authentication they will be sent an OTP to their phone, to submit as confirmation of their identity.
An OTP is not as vulnerable as a static password, as they can only be used once, and often for a limited time. The code cannot be reused for subsequent log-in attempts, and usually it must be entered within a certain period of time, or it will become invalid and the process must be started again.