NAS Security is between MME and UE whereas AS security is UE and eNodeB both serves the different purpose.
There are two ways to do this, i.e. Integrity and Ciphering.
For C-Plane: Integrity protection applied.
For U-plane: Integrity protection and ciphering.
In general term Integrity protection means, Receiver can verify that received message is the same as that Sender/Transmitter sends. Where as Ciphering/encryption means Transmitter/sender encrypts the data with a key and that key knows to only Receiver and no middleman can understand the data even if it gets it.
Let me know if have any comment...