A "slot" is usually the unit of pricing for a rented dedicated server; they are usually priced something like $N.NN per slot per month. So for example, if you want a dedicated server that will host up to 14 players, you get a 14 slot server rental. Pay the monthly fee, and you get access to the server - usually a control panel to start and stop the server, and FTP access to allow you to edit config files, upload map files, etc.
