CAN Bus is a data bus used in vehicles; here's a good site to start at

The car might be using the display of the HU to show stuff, or allow the HU to display information on a larger/more centrally placed display. Sometimes things like door chimes etc are going through the stereo...

Center speaker: a speaker located in the center of the car (or rather, the center of the dash), it's like the center speaker of a 5.1 system; reproduces stuff that is in common to both the left and right channel.

Bass reflex is a specific enclusure type/design, and yes, it's like a sub.

/Michael