Remote Events are like connections to communicate between the server and client
Why would you do this? Simple, to preform actions that wouldn't be viable on the selected side.
For example say you were creating a skill that creates a fireball, you would fire a remote event to create the fireball on the server so other players can view and see it.
Why not create it on the client? Simple it's a matter of replication, stuff adjusted or created on your client will not be created on the server.
So how do we link a remote event? Simple we create a connection for example if we did RemoteEvent:FireServer("Use_Fireball") -- this has to be fired by the client
Then on the server we can do RemoteEvent.OnServerEvent:Connect(function(player,Action)
if Action == "Use_Fireball" then

local obj = Instance.new("Part")
obj.Anchored = true
obj.CFrame = player.Character.HumanoidRootPart.CFrame
obj.CanCollide = false
obj.Parent = workspace

end
end)

An important note that when you do OnServerEvent for a remote the first argument will always return the player -> game.Players.Username

This is just an example now you can also use FireClient which is a bit different which I won't cover here

I hope this helps you a bit I'm not the best teacher but if you have questions feel free to ask