What is an Apse?
Hagia Irene in Istanbul (ancient Constantinople)
An apse is a rounded end of a building, most often a basilica or a church, but sometimes a private house or a Roman bath building. It just means that the end of the building is a semi-circle (half a circle) instead of being flat, like the wall of your house or school probably is.
Apses make a building look fancier, or more complicated. If you're going to be doing something important at one end of your building - like saying Mass at one end of a church, or judging court cases at one end of a basilica - then an apse makes that end of the building seem more important than the other end, so the people who are watching will know which way to look.
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).