The 1 Channel 5V Relay Module provides a single relay that can be controlled by any 5V digital output from your microcontroller. The relay is accessible using screw terminals and can handle up to 2A of current. A handy LED indicates the status of the relay.
An opto coupler is provided at Input so that a LOW at input will drive the Relay ON.
Relay terminals are extended with screw terminal.
Normally Open NO , Pole & Normally Closed NC contacts 
AC or DC load can be given to this contacts.For AC , phase is given to Pole and NO is connected to one end of load.Other end of load is Neutral.So when Relay operates phase reaches load through NO contact.