。
Want me to teach you a handy hidden command?
Literal
Convenient hidden-command teach-give shall-[question-か]?
A casual offer using ~てあげようか — the benefactive ~てあげる (doing for your benefit) combined with the volitional plus か ('shall I?'). The object marker を is dropped before 教えて, typical of casual speech. 隠しコマンド is a compound noun: 隠し (hidden) + コマンド (command, from English).