The phrase "beta period" refers to a testing phase when a product or service is made available to a limited number of users to gather feedback and identify any issues or improvements before a full release. Full definition
The online game has reached more than 3 million players since its open beta period, and current players will receive extra items when the game gets out of its beta state. (gamingtrend.com)
In the meantime, the studio announced it is extending the closed beta period by an extra two days. (inquisitr.com)
It should be noted, though, that «unlimited» isn't available during the current beta period. (jakonrath.blogspot.ru)