"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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conscienceforsale · 46-50, M

Its just something people ask so they can continue the conversation if you are doing something interesting. For example... if someone asks what are you up to and you say cooking, they would continue the conversation from there.