Complex sentence: The spin of composite particles, such as protons, neutrons, and atomic nuclei is usually understood to mean the total angular momentum, which is the sum of the spins and orbital angular momenta of the constituent particles.
Split into simpler sentences: The spin of composite particles, such as protons, neutrons, and atomic nuclei is usually understood to mean the total angular momentum. This is the sum of the spins and orbital angular momenta of the constituent particles.