What is a force and what units are used to measure force?

Force is what make a stationary object move or what changes the momentum of an object which moves at a constant velocity. momentum is the multiplication of the mass of the object and the velocity of the object. this is the definition of force from Newton's second law of motion.

F = ma - Newton's second law

force is measure in the unit Newton (N) mostly or it can be measured in

kgm/s² (killogram meter per square seconds)