How to monitor chat?

.Chatted event is what detects chat. However, you'll need to connect it to everyone:

Normal script in ServerScriptService:

function newPlayer(p)
    p.Chatted:connect(function(msg)
        if msg == "Pie is cool!" then
            -- Stuff you want to happen (use p to reference the player who said it)
        end
    end)
end
game.Players.PlayerAdded:connect(newPlayer)
for _,v in pairs (game.Players:GetPlayers()) do
    newPlayer(v)
end

game.Players.PlayerAdded:connect(function(player)
    player.Chatted:connect(function(msg)
        if msg=="Pie is cool!"then
            print(player.Name.." chatted \"Pie is cool!\"")
        end
    end)
end)

The Chatted event listens to everything a player says.