Imagery is description that uses the five senses of sight, sound, smell, taste, and touch.
Lee uses a good deal of imagery to make the town of Maycomb come alive for the reader, building a picture of it as a real place. She has a talent for finding images that stick in a reader's mind, such as when she sets the scene of the slow, hot summer in Depression-era Maycomb by providing such detail as the following:
Ladies bathed before noon, after their three-o'clock naps, and by nightfall were like soft teacakes with frostings of sweat and sweet talcum.
This conveys how hot it is in Maycomb in summer—the ladies bathe twice—as well as how slow life moves there—the ladies have time to bathe twice a day. It also communicates how soft these ladies are, inside and out, probably a bit overweight and also not prone to have been toughened up morally, when she compares them to "soft teacakes."
Lee uses sparer imagery to help characterize Atticus when she describes his office as follows:
Atticus’s office in the courthouse contained little more than a hat rack, a spittoon, a checkerboard and an unsullied Code of Alabama.
This suggests much about how Atticus might practice law. It just might be more important for him to build relationships over a checkerboard (which also suggests he isn't overburdened with work) than read the law code of Alabama (which is "unsullied" or little touched).
As you read, keep an eye out for other ways Lee uses imagery to build a layered picture of Maycomb life and to tell us more about her characters.