According to Google, “The Android Beta Program gives developers an opportunity to try out pre-release versions of Android. The program is currently targeted towards developers and is not suitable for use on your primary device (see known issues). Devices that you opt into the program will receive an over-the-air (OTA) update to the latest beta version of Android N.”
However as is the case with betas, while they are for the most part usable, they aren’t really meant to be used as a daily driver, which is why Google has also warned that taking part in the beta might see users receive updates that might not be 100% stable. “The updates that you’ll receive as a part of this program are unstable pre-release versions, and may contain errors and defects that affect your device.”
Also it seems that right now, devices that can take part in the beta include the Nexus 6, Nexus 9, Nexus 5X, Nexus 6P, the Nexus Player, and the Pixel C. The launch of the beta signup page follows the release of the developer preview of Android N, the next major Android build which is expected to see a release later this year.