what exactly does it mean when someone is in the concealed area? can they still attack people? can people still attack it?

701.19 says "Concealed" is a keyword and a characteristic which causes an object to come into play in it's controller's hidden area.

Rule 701.17 states that objects in the hidden area have the hidden caracteristic. It also states that a defending object must not have the hidden characteristic.

This is reiterated by rule 600.3a:

So in summary: no, hidden characters cannot be attacked.

600.3a is stating that if an attacking player controls a character that can attack a hidden character, that character may not attack directly if the defending player has any non-stunned characters, including in the hidden area. This is because those characters are valid targets for an attack.

Hidden characters usually cannot be attacked, unless there is an effect that states otherwise (such as Luke Cage or Coach).

600.3a is just kicking my @$$ today, no?

;)