PostgreSQL beta and release candidate releases are pre-release testing versions before the community makes a new release generally available. They are feature-frozen (i.e. no new features are added), and we release these to the public for testing before our final release. PostgreSQL beta and release candidate release are not meant for use in production systems.
Betas and release candidates are made available to the community in order to perform testing before the initial release. They are especially helpful for developers of platforms, drivers, tools, and utilities which depend upon PostgreSQL in order to prepare for changes in the new version. A release candidate should be identical to the initial release of a new major version of PostgreSQL, though additional changes could be made before the release is made generally available.
The stability of each PostgreSQL release greatly depends on the community to test the upcoming version with your workloads and testing tools in order to find bugs and regressions before the initial release. The quality of user testing helps determine when we can make a final release.
The PostgreSQL Global Development Group strongly advises against using beta versions of PostgreSQL in production installations or active development projects. While the PostgreSQL code is continuously subject to a number of automated and manual tests, beta releases might have serious bugs. Features are subject to changes that are backwards incompatible at any time during the development of the betas, and could possibly be removed altogether.
PostgreSQL 11 has been released. The first beta of version 12 is not yet available for testing. Beta testing of version 12 is likely to begin around June 2019.