Wiring up the SRD-05VDC-SL-C to an Arduino or Arduino-like microcontroller
Coil 1 and Coil 2 are simply the two terminals of the relay coil.

Common is the moving contact of the relay.  When the relay coil is not powered, the common contact is connected to the NC (normally closed) relay contact.  When power is applied to the relay coil, Common is connected to the NO (normally open) contact of the relay.