This expression is used when what someone offers to you is unexpected, or more than you expected. But in any case, you're very grateful to them for their favour. You can use it when you want to say "you don't have to do that"

뭘 이런 거까지 준비했어요? literaly means What did you prepare up to such a thing? It is an idomatic expression. it means "Why did you take the trouble to carry such a stuff?" Actually, "Thanks for your present!" 뭘 is colloquial form of 무엇을(of what) 뭐 is colloquial form of 무엇(what) ㄹ(을) is a accusative particle 이런 is colloquial form of 이러한(such, like this) 거까지 is colloquial form of 것까지 (up to thing) 거 is colloquial form of 것(thing,stuff) 까지 means "up to, until"