This describes - for any robot limb, joint or base - how many directions it can move in, and places at which it can move. It is commonly abbreviated as DOF.
The classic example is a robot arm. One which has a simple shoulder, and gripper could be said to have 2 DOF - the movement of the arm, and the action of the gripper.
An arm which has a rotating base, a shoulder and elbow, and rotating wrist with a gripper could be said to have 5 DOF. It is common though to exclude the gripper itself when referring to an arms Degrees of Freedom.
6 DOF is considered the kind of freedom a humanoid arm has - 2 for the shoulder - horizontal and vertical motion, 1 for the elbow, and 3 for the wrist - rotation, h and v.
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.