2FA is short for 2 factor authentication. It is another step of verification other than your standard username and password that makes unauthorized access to your account significantly harder.
This second step of verification is often provided by a third party service, credit/debit or access card a PIN number, SMS message and biometrics like fingerprints or retinal scans.
Multiple authentication factors significantly increase the difficulty of a bad actor getting access into a secured system as the requirement for multiple factors for authentication increases the requirements for access.
A good example of 2FA is the Google Authenticator which is used in many websites and services and adds an extra layer of protection above your username and password.