Using the Greek 2nd letter of the alphabet, beta describes the state of an application
during its development, when it has more or less full functionality but is still being tested.
During Beta, bugs can still occur so users of the software are made aware of this. New functionality can still be introduced though it's not that common at this stage.
Following beta, the application is considered almost ready for launch. It becomes a release candidate and then goes through a phase of extensive testing prior to release.
Google the popular search engine launches new apps at beta stage so it can test them live on users.