Question

What is a point mass?

Answer 1

a point mass is an object which has a mass that is indifferent or relative compared to the macrosystem
so since it has such a small mass it is considered as a point

Answer 2

A point mass is a point particle with non-zero mass. Basically you consider a particle as having no size, or structure, i.e. as zero-dimensional. So all the mass is at that single point.

It hugely simplifies calculations. For example, you can consider a ball with uniform density as a single point mass at the centre of that ball when you're dealing with gravity calculations.