Inertia is a measure of how difficult it is to change the velocity of an object. An object with a large mass has a large inertia, so it takes a bigger force to start it moving, to stop it, or to change its direction. That is why a loaded lorry is much harder to stop than an empty car.
Inertia also explains why passengers lurch forward when a bus brakes. The bus slows down, but the passengers tend to carry on at the same velocity because of their inertia.
We can measure it. The inertial mass of an object is defined as the ratio of force to acceleration, m = F ÷ a. A force of 10 N gives an empty trolley an acceleration of 5 m/s2, so its inertial mass is 10 ÷ 5 = 2 kg. The same 10 N on a loaded trolley gives only 2 m/s2, so its inertial mass is 10 ÷ 2 = 5 kg. The same force gives a smaller acceleration to the object with the larger inertial mass.