It means "what are you doing" or "what do you plan to do". When a friend asks you this, they're interested in your plans and perhaps planning to ask you to meet them tonight. If a security guard asks you this question, it means he thinks you're doing something illegal.

Timo is correct. "What are you up to?" is often a casual greeting or question between friends that means "What are you doing right now/lately?" and/or "What are your (immediate) plans?" Sometimes it's regarded as general, similar to "How's it going?": "Hey, there. What're you up to?" "Not much. You?" If an authority figure (police, security guard, etc.) asks, it is usually a direct question because they suspect you are somewhere you shouldn't be or may be doing something wrong. Saying that someone is "up to something" means they're planning or doing some kind of mischief.

This sentence means that "you have the decision about anything which will be done "