It is an event fired when a player talks, here's an example of it.
game.Players.PlayerAdded:connect(function(player) player.Chatted:connect(function(msg) print(msg) --Prints the message the player chatted end) end)
http://wiki.roblox.com/index.php?title=Chatted_(Event)
Here`s an example:
message="Hi" game.Players.PlayerAdded:connect(function(msg) if msg==message then print(message) end end)