As far as I know, 뭐라고 means "What did you say?" in an more informal way.

Seemopshcan is correct = 뭐라고 is "what did you say?" --> 라고 해요 is the grammar point for 'direct quotations'. 뭐 just meaning 'what'. 뭐라고? is just a condensed version and more casual. 뭘배워요? is 'what did you learn?" 배워요 = 배우다 --> To learn. 뭐 + 을/를 = 뭘 basically just meaning 'what' and attaching to the object marker 를/을. The 뭘 and 뭐 aren't really different. Although there may be a 'nuanced' difference, i have always remembered them as 'what' (in this context). Hope this helps