OTP is a one time pin (here: 6 digits) depending on a given password and the actual time.
- additional security when a user does a login
- take care of server requests (a request can be intercepted and repeated). As it depends on the date/time with OTP this isn't possible
- some sort...
Wikipedia: Google Authenticator
Most websites where you have to log in offer to protect your account by using a multi-factor authentication. Here you can use an app like Google Authenticator (GA) which shows a 6 or 8 digit number which you have to type in when you log in.
How it works: