Complex sentence: For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state, but in stable nuclei the possible lower energy states are all filled, meaning they are each occupied by two protons with spin up and spin down.
Split into simpler sentences: For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state, but in stable nuclei the possible energy states are all filled. Available energy states are already "paired", meaning they are occupied by two protons with spin up and spin down.