To succeed through hard work. Before zippers made getting into tall boots less of a chore, such footwear had leather attachments by which the wearer would pull them on (Western boots and some English riding dress boots still have them). Trying to raise yourself off the ground by pulling on your bootstraps sounds impossible . . . and it is (don't try it—you'll throw out your back). Therefore to pull yourself up by your bootstraps is to achieve your goals through as much hard work as levitating yourself would take.