Full Function

hook.Add("PlayerSay", "Dereinzigartige", function( ply, text )
    if text == "Hallo" then
        print("Hello")
    end
end)

PlayerSay_example.lua

--[[
    The hook add function adds a custom function to the said hook. That way our function runs everytime the hook is called.
    The hook add function has 3 parameters:

    1: The hook name ( for example PlayerSay )
    2: Your custom event name, which allows you to remove your hook later if needed
    3: the function to run when the hook gets executed
]]
hook.Add("PlayerSay", "MyAwesomeHook", function( ply, text )
    -- As you can see here the PlayerSay hook passes two parameters to your function:
    -- ply is the player object ( the player that said the text )
    -- text is the said text 
    
    if text == "MyCommand" then
        -- do stuff
    end
  
    -- With the return you decide what gets written into the chat, if you set it to "" the command won't be printed into everyones chat
    return ""
end)