"What's up with you/What are you upto?"

When I chat here every other person has this question inherently and as I am not a native English speaker,I don't know what to reply to this.So basically what exactly those phrases mean and what should I reply?or What is the reply they expect from me?

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Domking · 61-69, M

This question is an informal, colloquial way of asking "What are you doing?" or "What are you planning or desiring to do or enjoy?"

What's cooking is also the same question