What does double and triple gear shifters mean?
Double and triple refer to the number of chainrings (front gears) that you have. 

Here are the detailed service instructions, which allude to this fact:

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/NonSeries/ST-R221-R225/SI_6LU0A_En_v1_m56577569830606213.pdf