Can anyone help me understand multiplayer?

[BazTheHat]

Hi,

I've made my first Lua mod, Rain Pain, which effects implements when it is raining. At the moment it is only single player, and I'd like to make it multiplayer.

I've seen terms like self.client and self.server, but I simply don't know how multiplayer games work and which elements should be run server side, client side, or if it even matters. I also don't have anyone I can test the mod with in a multiplayer setting.

If anyone here has any experience with making mods work in MP and is willing to point me in the right direction, or even help test the mod out, I'd be very grateful.