What is meant by inertial mass,are all masses moving in gravitational field?

Answer 1

I believe that you're confusing "Intertial Mass" and "Gravitational Mass". There is in fact NO difference between the two except for the method of measurement.

Inertial mass is found by applying a known force to an unknown mass, measuring the acceleration, and applying Newton's Second Law, m = F/a. This gives as accurate a value for mass as the accuracy of your measurements. When the astronauts need to be weighed in outer space, they actually find their inertial mass in a special chair.

Gravitational mass is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass. This is typically done with some sort of balance scale. The beauty of this method is that no matter where, or what planet, you are, the masses will always balance out because the gravitational acceleration on each object will be the same. This does break down near supermassive objects such as black holes and neutron stars due to the high gradient of the gravitational field around such objects

Answer 2

an inertial mass is when it's not moving realtive to the frame of observer. mass is relative.
and yes, we all move through space, around sun ,in galaxy, in universe