Authentication is the concept or process to authenticate any user in some system. In authentication, we check whether particular user is authenticate to use that System, Organisation, any web portal or anything.
Authentication generally checked by the credentials of that user to check his/her authenticity.
If we see Online, our Authentication is checked at every step. Whether we have to login any portal, any social website or any banking website, login credentials are check ed at every step and according to username and password our identity recognised.
In nutshell,Authentication is mechanism to authenticate any user with the help of username and password.