arduino and relay battery power sketch

Projects General Guidance
3

What kind of "RELAY" ?
SSR = Solid State Relay
Coil = Magnetic Relay

What voltage is needed to turn on the relay?

I don't understand the 3 inputs:

  1. Power
  2. Signal
  3. Ground
    that are connected to the relay.

Usually, a Relay is connected with only two wires:
a) Power and a Pull-Low Output
or
b) Ground and Pull-High Output