Each frame has a dicepool for its actions, starting with 2 white dice representing the basic machine and the pilot's regular senses. You can expand this dicepool with up to 4 systems. No more than 2 systems of any given type can be equipped, but otherwise load-outs are unrestricted.
Yet, even without any systems present, a mobile frame can do several things using its white dice for these purposes.
Systems are restricted to their specific uses, but having one or more equipped unlocks certain special abilities that white dice cannot perform as well, if at all. Additionally, without any weapon systems present, a frame can only attack at 1 unit range, for hand-to-hand combat. Since MFZ is a game about attacking, such a frame would be at great disadvantage.