Question

what is constraint in motion

Answer 1

Can you please elaborate what you want to ask ?

Answer 2

When different parts of a system can move relative to each other, there are sometimes links that allow only certain types of motion.
It is also the case when something is rolling on a base.

Then the number of degrees of freedom of the system decreases because of these constraints.

Let's take a wheel moving in a vertical plane.

It has 3 degrees of freedom : abscissa and height of its centre of mass plus angle between the vertical and a special spoke of the wheel.

If the wheel is touching the ground (constraint), there are only 2 degrees of freedom left : abscissa and angle, because height becomes fixed equal to the wheel's radius.

Now, if the wheel is rolling, then angle and abscissa are proportional, leaving only 1 degree of freedom.

The more constraints, the fewer degrees of freedom for your problem.