What is meant by the following statement? "inertia is directly proportional to mass."

It means that the greater the inertia, the greater the mass, and the less the mass, the less inertia, is what is known as Newton's first Law.

Explanation:

Newton's first law states that every body continues in its state of rest or in a uniform and straight line movement, as long as there is no external force that modifies it. The inertia is directly proportional to the mass, which means that the greater the mass, the greater the inertia, the smaller the mass, the less inertia.