NAS (Non-Access Stratum) protocol is responsible for communication between the UE and the mobile core network in the control plane.

In turn, NAS signalling connection refers to the logical communication channel established between the user equipment (UE) and the mobile core network to exchange signalling messages.

The access network (e.g. eNB in 4G and gNB in 5G), transparently transfers NAS messages exchanged between the UE and the mobile core network.

NAS signalling connection operates in the control plane. And it is separate from the user plane connection, which is responsible for the actual data transmission between the UE and the mobile network.