What does radical stability mean?
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.