It is possible to read and send mail from your email accounts by logging in in your web browser, rather than using a mail program. This may be useful, for example, if you are using a public computer, or somebody else's device.
The email address you want to use is the user name. The password for that email address is the log in password.
When you log in you may see an option to pick a preferred mail client. You can select one to be the default client. Each client has different features in terms of appearance, but are still providing the same function - to see what's in your mailbox!
After you are done it is a good idea to log out, and especially important for account security if you are on a public computer, perhaps at a library, for example.