This stamp was issued to commemorate the Bicentennial of the US Constitution. The stamp pictures Independence Hall, since it was drafted and signed in that building.
From the U.S. Stamp Gallery website:
Pennsylvania played a major role in the Revolutionary War, continuing its leadership in the Colonial period. Benjamin Franklin, the colonial agent in Great Britain, as early as 1766 opposed the Stamp Act before the House of Commons. Following the outbreak of the war, Pennsylvania troops took part in many campaigns. The state created its own navy in 1775 to defend its shores. The battles of Brandywine, Fort Mifflin, Germantown, and Paoli were fought within the state. Valley Forge is located 25 miles west of Philadelphia. Delegates to the Constitutional Convention in Philadelphia in 1787 drew up the U.S. Constitution, which Pennsylvania became the second state to ratify.
From the National Postal Museum website:
Pennsylvania's past is America in microcosm, containing many elements found in American history as a whole. Philadelphia played a singular role in the creation of the United States, serving as the site of the First and Second Continental Congresses, the signing of the Declaration of Independence, and the Constitutional Convention. Pennsylvanians James Wilson and Gouverneur Morris were considered two of the principal architects of our treasured Constitution.
Founded as a 'holy experiment' out of William Penn's desire to establish a haven for persecuted fellow Quakers, the Keystone State is now highly diverse, with its economy fueled by iron and steel mills, coal mines, and agricultural activities aided by unusually fertile land.
The Postal System issued the Pennsylvania statehood stamp as part of its philatelic commemoration of the bicentennial of the United States Constitution. Richard Sheaff of Needham Heights, Massachusetts, designed the stamp.