'~을/를 했다‘ is the past form of '~을/를 하다'. It can be used to describe some activity you have done.  '뭐,' the reduced form of '무엇’, is more frequently used in a colloquial conversation.

무엇을 했어요?

메이: 어제 무엇(을) 했어요? (무어슬)

민수: 책(을) 읽었어요. (채글 일거써요)

메이: 무슨 책(을) 읽었어요?

민수: 소설책(을) 읽었어요.  

메이: 재미있었어요? 

민수: 네. 아주 재미있었어요.

  May: What did you do yesterday?

Minsu: I read a book.

  May: What did you read?

Minsu: It was a novel. 

  May: Was it interesting? (or did you like it?)

Minsu: Yes, it was really interesting.

Key words and expressions

-어제 yesterday

-읽었다 past tense of ‘읽다’ 

-읽었어요 honorific expression of '읽었다‘

-소설책 a novel

-재미있다 (to be) interesting

-재미없다 negative form of '재미있다‘

 Grammar Tip                   past tense      honorific form

-(책/신문/소설)을 읽다        ~ 을/를 읽었다   ~을/를 읽었어요

-(잡지/시/만화)를 읽다

Colloquial Expression

-어제 무엇을 했어요? -> 어제 뭐 했어요?