It's probably a virtual reality made to please the Pokemon inside.
Poké Balls are specifically constructed for Pokémon capture, transport and training. As well as being physically difficult to escape from (as they seal tightly shut as soon as a Pokémon is taken into them) the environment of a Poké Ball is designed to be attractive to Pokémon also; according to Lucian of the Sinnoh Elite Four, weakened Pokémon instinctively curl up tight in an attempt to heal themselves, an action that the environment of the Poké Ball encourages. Furthermore, while it is not known how a captured Pokémon perceives their time inside their Ball, the device is said to replicate a "Pokémon-friendly" environment that is "designed for comfort". All of these factors strongly discourage Pokémon from escaping their Balls. In the manga, Bugsy refers to his "capture net" as being the net that is supposedly inside a Poké Ball, but visible and already deployed. According to Kurt, this invisible net captures and physically stores a Pokémon.-Bulbapedia