Google Authenticator is a software-based authenticator by Google that implements two-step verification services using the One-time Password Algorithm (OTP) for authenticating users of software applications.
When logging into a site supporting Authenticator, the Authenticator generates a six-digit one-time password which users must enter in addition to their usual login details.
You can also refer to this YouTube video for more details: https://www.youtube.com/watch?v=rbo1iFLMZyI