A buttress is a big pile of stone that keeps a building’s walls from falling down. If you build a building out of stone, and then put on a stone roof with a barrel vault, you have a problem: the roof is so heavy, and it presses down so much on the walls, that the walls get pushed outward and fall down.
To keep the walls from falling down, you can make them very thick and strong, with only little tiny windows. But then your building will be very dark (especially since there were no electric lights in ancient Parthia or medieval Europe!).
A better approach is to use groin vaults to direct most of the weight of the roof to specific places on the walls. Then you only have to brace those places, and in between you can have big windows to let the light in. Those braces are called buttresses (BUT-ress-is).
People used buttresses from the days of the Roman Empire and the Parthian Empire through into Medieval Europe and the Islamic Empire. Around 1150 AD, in medieval Europe, architects began to also use a new kind of buttress called a flying buttress. With a flying buttress, you could make even bigger windows than with ordinary buttresses.
Learn by doing: find a building in your town that has buttresses
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. For younger kids.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books for kids, and with good explanations of most architectural terms.
City: A Story of Roman Planning and Construction, by David Macaulay (1983).