How Do I Add Mods, and change the world to my minecraft server?

I use HostHorde which isn't the ordinary minecraft server thing. I see something on there about world but its not like a click to open a folder thing, it's more like a search query. Please tell me how to upload a specific world I have on single player and also how to add mods to my server. It would also help if you could explain plugins.

Andrea 25.02.2018

I'm not sure about the map but for mods create a folder in you minectaft folder via typing %appdata% in the search bar on your computer and clicking minecraft then create a folder titled mods and add the downloaded mods to the folder

Ethan 25.02.2018