EDIT: since you don't know what this is supposed to do, lol.
Its a custom Hint message GUI, for when a player joins, it shows Player name and "joined the server!"
and after 5 sec it is gonna disappear.
local frame = script.Parent.Parent
local TextLabel = script.Parent
frame.Visible = false
game.Players.PlayerAdded:connect(function(Player)
frame.Visible = true
TextLabel.Text = Player.Name.. " joined the server!"
wait(5)
frame.Visible = false
end)

2 comments
You need to be more specific. What doesn't work, which part of the code doesn't execute?
More information is needed. What doesn't work? Any output? Where is the script? What type of script is it?