A user story is a sentence or a phrase that explains how a user will interact with a specific feature of your system from their perspective.
An example of a user story is:
“As a user, I want to log in using my Google account”
What the user story tells you is that there is a user who needs to log into your application or system and that you must enable Google logins to allow them to log in using those credentials. What this means is that a user story guides the requirements to be implemented to meet the desired outcome.