Here is a fairly typical mid-price accordion made by Bugari, a respectable Italian company producing  finely made instruments.  
•    Bass buttons:
This is a "72 bass" instrument, meaning it has 72 buttons on the bass side  These are in the usual  layout, with a row each of: bass notes; counter bass notes; and chords of: major, minor, 7th and diminished types.  A 72 bass insturment gives all the standard notes and chords, but has fewer duplications than a "full size" 120 bass button board, so occasionally more “jumps” are needed when playing in less common keys.  
•    Bass Couplers (aka registers or switches):

These are the two flat rectangles between the bass buttons and the bellows.  
These give two different bass-side sounds, and one will be “lighter”, to balance quieter playing on the keyboard.