Sightseeing is what you do when you travel somewhere and spend time visiting museums, sites, or neighborhoods. You might dream of sightseeing in Paris someday.
Sightseeing is an activity that's almost always done in an unfamiliar place, and usually during a vacation. Going sightseeing in the city where you live isn't impossible, though—you could spend a day taking tours, visiting historic sites, and looking at your city the way a tourist would. Sightseeing has been in use since the 19th century, from the idea of "seeing the sights."