Testing a Relay Card

I don't know what this Dry Contact Input means. I've tried Googling but I can hardly understand their technical explanations.

I've read somewhere that a Dry Contact is also a Digital Input but some other website says that it is not the same. I've also read about putting pull-ups or pull-down resistors onto them. I will attach my Relay Card and its diagram.

Thanks in advance.

Also, if you have any program for this using Arduino Uno, would you mind sharing it? Thank you very much!

I think dry contact means the input isn't connected to anything else in the circuit. So just a relay winding or just its switched contacts - so that any circuit can use it without needing a common ground or such.

"Dry contacts" means a given set of switch or relay contacts, to be wired to some form of digital input, has no external voltage on the contacts. That means for most digital inputs one must provide contact sensing voltage, either internally from the digital input circuitry (internal pull-ups) or by means of external pull-up resistors.