For the first time in the long running series, probably due to its online nature, the next Fallout is getting a beta ahead of its release in November. The beta or B.E.T.A. (Break-it Early Test Application) is scheduled to kick off in October and will roll out to selected users who pre-ordered on Xbox One and "grow over time as we prepare for launch".
The planned beta window was announced via a community post
over at the Bethesda forums which does include confirmation that everyone who pre-orders Fallout 76 will get access to the beta. The planned, and gradual rollout is most likely due to server testing and to fine tune ahead of the likely millions of people planning to dive into post-apocalyptic West Virginia this holiday season.
In terms of the game itself, the post also mentions that "things are still changing and being iterated on", meaning that several mechanics or multiplayer rules are probably still being fine-tuned ahead of further gameplay reveals.