A web application is said to be stateless when it doesn't store any information to remember a user's last visit.
Use Cases and Examples
This means whenever the frontend requests or sends data to the backend it needs to inform the backend who that data is from. The frontend has to store extra user information that can be sent to the backend acting as some form of key.
When it comes to stateless applications the responsibility of remembering "who is who" is pushed to the end-user or the frontend side of things.
This could be beneficial when it comes to scaling since the backend does not need to know the state of the user and the frontend runs on the client's device, this means in situations where one backend server is overloaded another can pick up the tab and handle the client's request.
Here is another article you might like 😊 "What Is A Backlog?"