Inertia is the property of a body such that it tends to keep moving in a straight line unless forced to change.
Inertial mass is in a sense the amount of matter in a system with that property.
Because F=ma you can say it's the ratio of force you need to apply to a body to get a certain acceleration.
Note that Gravitational mass is due to another property of matter, that is it's tendency to attract other matter.
The Gravitational constant G is defined in such a way that in most applications we can consider Gravitation mass to be equivilent to inertial mass even though these are two quite distinct properties of matter
