Simple Definition:

Inertia is the tendency of an object to resist changes in its motion [movement or direction]. It is a property of matter that causes objects to either stay at rest or keep moving in a straight line at a constant speed unless acted upon by an external force. For example, a ball will keep rolling unless someone or something stops it; a car will keep moving at the same speed unless the brakes are applied.

Very Simple Definition:

Inertia is when things want to keep doing what they’re already doing. For example, a ball wants to keep rolling and a car wants to keep driving unless someone or something stops them.