To Be Up To!

Hey!

To be up to do something is to be good enough to or to be capable of doing a task or a work. 

For example: She is not up to do this. I think it is better employ another one.

Another meaning and I believe the most common, too, is like to do something.

For example: If a father sees his daughter playing on the ground and she suddenly stops. Probably, he would be curious to know what is going on, so, he would ask "What are you up to?!" it could be replaced by "What are you doing!?/What is going on!?" 

It can be used when you wanna know if someone has plans, like: Hey! What are you up to this weekend?! ; )

Got it?!

~~

Have you heard "What is up"?!

It is a slang and a way to ask "What is going on in your life?!" or "What has been happening?!" :) Hope you got me