Plugins and mods.
Paper and Purpur run plugins, and Fabric, Forge, and NeoForge run mods. Your server's page finds them for you.
Open the tab.
On your server's page it's called Plugins or Mods, depending on the software. Vanilla runs neither.
Pick one.
Search or browse. Only what fits your server's software and Minecraft version shows up.
Install it.
The page says which version you'll get and what else it needs, and installs that too.
Restart the server.
Nothing changes until it next starts. The page says what's waiting and has a button for it.
Good to know.
- Friends install nothing for a plugin. A mod that changes the game has to be on everyone's computer too, at the same version.
- Have a jar from somewhere else? Press Upload a jar on the same tab. The page can't check that it fits, or update it later.
- When a newer version fits, its row offers the update.
- The listings come from Modrinth. If the server won't start after adding something, its page says which one it was, when the log says so.