When you get an eyeful, you get a good, long look at something or someone. Climbing to the summit of a mountain can give you an eyeful of the beautiful valley below.
A complete view of anything can be described colloquially as an eyeful, but for some reason people usually mean "a good look at a naked person" when they say it. Your mom might say, "A streaker ran nude through the football game, and boy did we get an eyeful!" Occasionally, the word means "a good-looking person," as when someone says, "Your cousin's really an eyeful!" This casual word has been used since the late 18th century.