Shield stacking headers are the second most important part in the Arduino world (After the Arduino itself!) These pins are inserted through an Arduino Shield in place of standard breakaway pins and allow additional shields to be stacked on top of each other, meaning multiple shields on one Arduino. For example: You can stack an Adafruit LCD shield on top of one of our Development Boards or you can stack an Adafruit GPS Shield on top of an Adafruit Motor Shield. Very Useful!