Autobuilds are nightly builds of the latest code. They have the latest features, bugfixes and assets but they could be less stable than regular releases.
If you "installed" Xonotic by downloading and unpacking the zip, you can update a regular release to autobuild using the scripts located in misc/tools/rsync-updater (use update-to-autobuild.bat if you're one Windows, update-to-autobuild.sh on Linux and Mac). You can also download an autobuild and various other builds as a zip from https://beta.xonotic.org/autobuild/ - username is xonotic and password g-23.
It's best to make a copy of the original Xonotic directory first in case you wanna go back to using the release but you can also go back by using the update-to-release scripts. Releases and autobuilds should be able to coexist (they will share the same config though, unless you use something like -userdir).
Autobuilds (and git builds) are compatible with the same servers as the latest release. When you connect to a server, Xonotic downloads the gamelogic from it (and runs it sandboxed) so you always use the same version as the server. Autobuilds therefore only affect the gamelogic of local games, the engine (rendering, input, ...) and assets (models, sounds, ...).