Every object resists changes to its motion. Inertial mass is a measure of how difficult it is to change the velocity of an object. That includes making it start moving from rest, speeding it up, slowing it down or changing its direction. A lorry has a much larger inertial mass than a trolley, so it is far harder to get moving or to stop.
Inertial mass is defined as the ratio of force to acceleration: inertial mass = force ÷ acceleration, or m = F ÷ a. This is Newton's second law rearranged. If the same force is applied to two objects, the one with the larger inertial mass has the smaller acceleration.
As a worked example, a force of 12 N gives an object an acceleration of 3 m/s². Its inertial mass is 12 ÷ 3 = 4 kg. Pushing a 6 kg object with the same 12 N gives an acceleration of 12 ÷ 6 = 2 m/s², which is smaller because the inertial mass is larger.