Packages in AS3

So long story short, ive been gone a long time from designing in general. ive come back to flash and…well…this AS3 thing isnt going as fast as id hoped lol.

so my ? is, what are these packages…sometimes when i look for begginer tutorials, they all start with these packages and all this other stuff infront of the code like… like

import flash.events.MouseEvent;
import flash.net.URLRequest;
import flash.display.MovieClip;
import flash.display.Loader;
import flash.events.Event;

etc…

what IS all this?!?!?