Inertia is simply the resistance of an object to change in velocity (speed with a direction).  Inertia is directly related to the mass of the object (dumbbell, barbell, weight stack).

We must overcome inertia every time we try to move a load from a dead stop or when we try to change the direction of a weight that is moving.  Inertia is different than momentum, which you can learn more about and it’s training application HERE.