Authentication is how the apps you use know that you are who you say you are: through something as simple as a password, or as complex as biometrics.
Accessing your apps and data on the web relies on identity: only you should be able to see your stuff! The most popular authentication method is username and password, but schemes like single-sign-on and magic links are getting more popular. On the backend, authentication can work through cookies, session tokens, and/or other combinations of esoteric cryptography.