Inertia is the quality a mass, or any object, has that keeps it still, if it is not moving, or in motion, if it is in motion. The relationship between mass and inertia has much to do with the first two laws of physics by Isaac Newton, which are that an object at rest will tend to stay at rest and an object in motion will tend to stay in motion.
In order for the force of inertia to be overcome, a certain amount of force must be applied. Objects that are heavier or traveling at higher speeds typically need more inertia to change their direction or speed than objects that are lighter or hardly moving.
There are other more advanced concepts concerning inertia. One such concept is the idea of the moment of inertia, which refers to rotational inertia. This type of inertia is found in objects that have rotational motion, a type of motion that occurs when a mass is spinning on an axis or traveling in a circle.
In linear motion, the moment of inertia is simply described as force being equal to mass times acceleration but, in angular motion, the angular motion is equal to the moment of inertia multiplied by the angular velocity. Inertia and mass and how they relate to each other appear in a multitude of other mechanical concepts within the field of physics.