What is OAuth?
OAuth is a sign-in standard that lets you give an app limited access to your account without ever giving the app your password.
When you click "Sign in with" a large provider and see a screen asking whether an app may access your mail, that is OAuth. You approve the request with your provider, and the provider hands the app a token. The token grants only the access you agreed to, and you can revoke it later from your account settings without changing your password.
OAuth and email
Some email providers now require OAuth for third-party apps and have switched off, or are restricting, sign-in with a plain username and password. Others still allow a password, often an app password created for one specific app. Which applies to you depends on your provider and, for business accounts, on what your administrator has enabled.
OAuth and inboxr
inboxr does not offer OAuth sign-in today. It connects to a mailbox using an IMAP and SMTP host, a username, and a password or app password. If your provider only allows OAuth sign-in for third-party access, a password connection may not work with that mailbox. The Test connection button on the mailbox form tells you before you save, and your provider's help pages explain which sign-in methods it allows.