말해 줘 is composed as 말하(다) + 여 + 주(다) + 어 말하다 : to tell 여 : connecting particle used after 하다 verb. 주다 : (auxiliary verb) to do something for someone. 말해 줘 is contracted form of 말하여 주어. 말해 줘(informal) means "tell me (for me)" 말해 줘요(a little formal) 말해 주세요(formal) 말해 주십시오(more formal) 말씀해 주시기 바랍니다.(much more formal) For auxiliary verb, refer to https://www.italki.com/question/365005

So basically 주다 means 'to give'. when added to a verb, it can have the meaning of doing something for someone. 말해 - speak 말해줘 - tell me 해 - do it 해줘 - do it for me you can use it also in different situations. 말해줄게 - i'll tell you (for you) 보여 - show 보여줄게 - i'll show you ask me more questions if it wasn't clear