At this time, the combat functionality in RPG in a Box is very limited. The plan is to expand the current turn-based tactical system (and to implement other combat systems) once I've completed the upgrade to Godot 3.0. I expect this to be sometime in early 2018. In the meantime, refer to the Battle Editor and Enemy Editor pages for more information about the functionality that's currently available.
Turn-based tactical is the default combat system for RPG in a Box. In the current implementation, the player and enemies can move to a tile within their movement range (and optionally attack) during each turn. Future implementations will build out other common aspects and take inspiration from tactical RPGs like the Shining series and some of the older Ultima games.