What is inertia?

Inertia of a body is its property due to which it resists change in its state of rest or of uniform motion.

A passenger in a bus moves outward when a bus takes a sharp turn because he resists change in his state. When brakes are applied we feel a jerk that is because the lower part of our body is in rest with the seat in vehicle but our upper part of body is in motion and resists change due to which we move forward when brakes are applied.