Radical stability refers to the energy level of the radical.
If the internal energy of the radical is high, the radical is unstable. It will try to reach a lower energy level.
If the internal energy of the radical is low, the radical is stable. It will have little tendency to react further.
Free radicals have only 7 electrons in their valence shell. They are higher in energy than atoms with 8 valence electrons.
Carbocations are also electron-deficient species. They are even higher in energy, because they have only 6 valence electrons.
Electron-donating alkyl groups stabilize carbocations. They also stabilize free radicals.
So the order of stability of free radicals is the same as for carbocations:
This means that tertiary radicals are most easily formed, and methyl radicals are least easily formed.