Inertial mass is a measure of how difficult it is to change the velocity of an object. This includes getting it moving from rest, speeding it up, slowing it down or changing its direction. A heavy lorry has a large inertial mass, so it is hard to start and hard to stop.
Inertial mass is defined as the ratio of force to acceleration: m = F ÷ a. This comes straight from Newton's second law, F = m × a. For example, a force of 20 N that gives an object an acceleration of 4 m/s2 means its inertial mass is 20 ÷ 4 = 5 kg.
For the same force, an object with a large inertial mass has a smaller acceleration, so its velocity changes slowly. An object with a small inertial mass is easy to accelerate.