The 13 stripes on the American flag represent the original 13 colonies that declared independence from Britain in 1776. The stars represent the number of states in the union. Their number has grown from 13, and became 50 when Hawaii joined the United States in 1960.
The first official U.S. flag, known as the Betsy Ross flag, featured 13 stars and 13 stripes. When two new states joined the union, the flag was changed in 1794 to include 15 stars and 15 stripes. However, increasing the number of stripes for every new state was not practical, and President James Monroe accepted a bill on April 4, 1818 that went back to the 13 stripes unchanged but added a new star for every new state.