Matter has the property of inertia, which is the resistance of matter to be set in motion or to change its uniform motion.
Inertia is greater the greater the mass of a body. For example, a tennis ball has little inertia, and it takes little to set it in motion, while the inertia of a lorry is great.
When a body has greater inertia, it is necessary to apply a greater force to produce a given acceleration in the motion of the body.
The following visual simulates the movement of two cars. You can modify their parameters to understand what inertia is.