What does two-step authentication mean?

Two-factor authentication is a method of identifying a user in a service (usually on the Internet) by requesting authentication data of two different types, which provides a two-layer, and therefore more effective, protection of the account against unauthorized intrusion.Open the Google Account page.
In the navigation bar, select Security.
Under "Sign in to your Google Account", click Two-step Authentication Start.
Follow the instructions on the screen. 
Alternatively, use the yubikey key https://rofldna.org/yubikey-makes-online-security-easy/ .

Two-step authentication provides better account protection - attackers won't be able to access your account even if they know your password. When this feature is enabled, two components are used to sign in:
What you know (e.g., your password);
What you have (for example, your phone).