Each motorised joint normally adds one degree of freedom (DOF). Three of them are needed to reach a point in space, three more to set the orientation of the tool, which is why six-axis arms are the industry default.
A seventh joint makes the arm redundant: it can reach the same point in several different poses, useful for working around an obstacle or inside a machine. A SCARA arm has four DOF because it never needs to tilt its tool. A human arm from shoulder to wrist has seven.
More DOF means more flexibility but also more cost, more mass and harder control, since the software must choose between many possible joint configurations for the same task.
