# holonomic and non holonomic constraints in layman's term

I'm new to robotics and I've been reading some slides online regarding motion planning. Due to my lack of knowledge in mechanical engineering, I'm having a difficult time understanding what holonomic and non-holonomic constraints mean.

I saw a post here and it says Holonomic system is when a robot can move in any direction in the configuration space, and Nonholonomic systems are systems where the velocities (magnitude and or direction) and other derivatives of the position are constraint.

It seems like holonomic system differs from holonomic constraint. What is holonomic constraint and when do we need it? What is non-holonomic constraint and when do we need that?

A constraint on the $k$ coordinates $r_1,r_2,...,r_k$ is holonomic if it is an equality constraint of the form $$g(r_1,...,r_k)=0,$$ and nonholonomic otherwise. These sorts of constraints arise frequently in mechanical systems (e.g. when deriving Euler-Lagrange equations of motion). A holonomic system is one that is subject to holonomic constraints, and a nonholonomic system is one that is subject to nonholonomic constraints.