'Tell me about yourself' is an instruction to give an overview of yourself. The information you give will vary according to the situation. For example, what you say on a date will not be the same as what you say in a job interview. 'What about you?' is a question and it means you should give the same information that the asker just gave. e.g. "I'm going to order the chicken. What about you?" - What are you going to order? "I'm from the UK. What about you?" - Where are you from?

1️⃣What about you? (¿Y tu? O ¿Qué hay de ti?) Suele utilizarse para que contesten la misma pregunta que te hicieron mayormente al conocerse por primera vez o reunirse después de un tiempo. 2️⃣Tell me about you (cuéntame de ti) es muy común cuando vas conociendo mas a alguien o cuando te entrevistan)