It means "What have you been doing recently."
Typical responses could be:
"I have done nothing for a while now. I am taking a break from work."
"I've done nothing special. Life is the same as usual."
"I've started a new and exciting job since we last met."
If a parent says this in an accusing tone, the response by the child might be:
"What do you mean? I've done nothing wrong!"
The list of possible responses is endless.
Can I reply saying "I've been working on my project". I mean can I use present perfect continuous tense to reply?