Google Authenticator is a software-based authenticator by Google. It implements multi-factor authentication services using the time-based one-time password (TOTP; specified in RFC 6238) and HMAC-based one-time password (HOTP; specified in RFC 4226), for authenticating users of software applications.[5]

When logging into a site supporting Authenticator (including  Google services) or using Authenticator-supporting third-party  applications such as password managers or file hosting services, Authenticator generates a six- to eight-digit one-time password which users must enter in addition to their usual login details.

Google provides Android,[6] Wear OS,[7] BlackBerry, and iOS[8] versions of Authenticator. 

An official open source fork of the Android app is available on GitHub.[9] However, this fork was archived in Apr 6, 2021 and is now read only.[10] Current software releases are proprietary freeware.[11]

(From Wikipedia, the free encyclopedia)