What does NC mean in this pinout diagram for an ESP8266?
Since it's labelled as a type P, it should be a power input pin for RTC. 

Extract from datasheet:



It appears that if you keep RTC powered on, it will keep basic settings intact while in sleep. 

Even though complete info is not present, I think pin 5 can be fed 1.1V externally to keep the RTC running even while your 3.3V supply is off.

So, usually, you aren't supposed to connect anything to it but you can connect a 1.1V (or maybe 1.1V is the minimum voltage required) power source to keep RTC running so that wifi configuration remains intact.