Well, I think you're mistaking a bit . Let's say that you got the 5 V version, you plug the 5 V from arduino into vcc , gnd to gnd and the the input pin to one of the entry on the board. Current needed to make the relay toogle is about 15 or 20 mA which can be given by the arduino. On the other side, you got your power supply of 24 v and the device you plugged in .
So to resume, you activate to high on entry from the arduino which activate the relay who can let the current flow through it.
Refer to this link, there's two pictures that explain surely better than me 
: https://www.hobbyist.co.nz/?q=interfacing-relay-modules-to-arduino
However, this is for a 5 V version as I told you because this a lot easier than the 24 version. For the version, you will have to play with mosfet which is more work.