Google Authenticator is a smartphone app (available for Android & iPhone) that makes it easy to set up two-factor authentication for your online accounts. This provides a second layer of security, in addition to your username & password.
Google Authenticator provides a six-digit code that refreshes every 30 seconds, and can only be accessed via your smartphone. Even if someone guessed your username & password, they would need to be in physical possession of your phone in order to access your account. This is one option (out of several) in which you can set up two-factor authentication for any of your online accounts.