Zero moment point (zmp) is one of the most used concepts in humanoid robots locomotion and walking. One can check the so called "static stability" of a humanoid robot by projecting its center of mass (CoM) and checking if it lies within the support polygon. Such measure is not sufficient though, when the robot joints are moving and/or it is subjected to forces (e.g., moment of inertia).
To deal with this, the concept of zmp is introduced. It basically take into account robot motions, ground reaction forces, etc, and provides criterion to maintain robot stability during motion.