Beta versions of games are the late stages of pre-release testing wherein the developers can get a better idea of how the game runs on a larger variety computers so further optimisations can be done and where more faults can hopefully be found and addressed. Also, Gameplay and map feedback are often analysed further at this stage. Not every game has public beta releases. Many developers keep this private all the way through to release.
Please note, beta programs often have user policies that prohibit discussion away from official forums which relate to game specific elements. Be aware of this before posting any information.