Inertia is a resistance to change in energy. The more massive something is, the harder we have to push to change its speed or direction.
There is a time delay in the energy transfer.
The fact that inertia only exists in matter in the open state is a huge hint as to its nature.
Energy transfers to and from matter in the closed state happens instantaneously because matter in the closed state is very small. It happens faster than can be registered.
However, the time it takes to transfer energy to and from matter in the open state can be registered. It takes more time to transfer energy onto an electron than it takes for a photon to cross it.
Energy transfers are about readjusting sizes of particles. For matter in the closed state, this happens faster than it takes a photon to cross an electron. For matter in the open state, it takes more time.
For an object to change its speed or direction, all its particles have to change size. For large objects, this requires a lot of energy and time.
Inertia is the time delay registered when energy of particles in the open state are changed.