What does that mean exactly?

All of a sudden my Rails app's attempts to send via SMTP started getting rejected until I enabled "Less secure app access". It would be nice if I knew what was necessary to make the access considered "secure".

Update: Newer information added to this article (as well as elsewhere) leads me to believe that it is specifically sending password directly as authentication mechanism which was/is no longer permitted.

This is the note that has since been added on this page, which clarifies this point:

To help keep your account secure, from May 30, 2022, ​​Google no longer supports the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password.