Debian testing is the current development state of the next stable Debian distribution. It is also made available under the code name of the next stable release, Bullseye is the current testing codename.

What is Debian used for?

Debian is an operating system for a wide range of devices including laptops, desktops and servers. Users like its stability and reliability since 1993. We provide reasonable default configuration for every package. The Debian developers provide security updates for all packages over their lifetime whenever possible.

Should you test Debian?

To get the most updated packages but still have a usable system, you should use testing. Unstable should be used only by developers and people who like to contribute in Debian by testing the quality and stability of packages, fixing bugs, etc. … So I personally recommend Debian Testing, not Sid.