what is the difference between :FireServer and :FireClient?

i am starting to use filtering enabled for my games now, as roblox will soon be blocking games without it in the future. i was able to grasp the basics of RemoteEvents. but then i came across :FireClient, instead of :FireServer. i assumed it made it so it was only possible for the client to view it? i tested it out with "OnClientEvent" and nothing happens. i just want to know what it is, what it's used for, and how to use it. because when i tried testing it out, nothing happens. no errors in the output either.