I have been following "Lie Algebras in Particle Physics: From Isospin to Unified Theories". In the textbook, the chapter on tensor operators defines them as operators that transform under commutation with generators as $[J_a, O^s_l] = O^s_m[J^s_a]_{ml}$, where $J_a$ are the generators, and $O^s_l$ is the tensor operator. My question is, where does this definition come from?
Sidenote: Any other recommendations for textbooks covering similar topics would be highly appreciated. I am finding this set of notes a bit difficult to follow.
