<http://postimg.org/image/76newirnv/>

For application proxy, 

Firstly, what does it mean "A proxy is something that acts on your behalf"? Can anyone give a more detail explanation about proxy? I read a number of papers but still bit confused

Secondly, how can application proxy know if the incoming data is safe or not?

Thirdly, how does a proxy function if i am going to send packets to external network