What is meant by inertia?

Continuing in an existing state. Resistance to change.

According to Newton's first law of motion a body continues to be in the state of rest of uniform motion along a straight line unless it's acted upon by an external force to change its state. This means a body on its own cannot change its state its state of rest or state of uniform motion along a straight line. This inability of a body to change by itself its state of rest or uniform motion along a straight line is called inertia of body.