Objects resist changes to their motion. An object at rest stays at rest, and a moving object keeps moving at the same velocity, unless a resultant force acts on it. This is called inertia. Inertia is a measure of how difficult it is to change the velocity of an object. Velocity includes direction, so it is just as hard to change the direction of a massive object as its speed.
The more massive an object, the greater its inertia. A trolley of mass 5 kg has more inertia than a trolley of mass 2 kg, so the same push gives it a smaller acceleration. It is harder to start it moving, harder to stop it and harder to turn it.
Newton's Second Law lets us define inertial mass. It is the ratio of force over acceleration: inertial mass = force ÷ acceleration. If a force of 12 N gives an object an acceleration of 3 m/s2, its inertial mass is 12 ÷ 3 = 4 kg. An object with a large inertial mass needs a large force to give it even a small acceleration.