Imitation is the repetition of a musical figuration. The figuration can be varied with fragmentation, transposition and/or inversion, and the intervals and rhythms of an imitation may be exact or modified. In a poly-phonic setting, successive repetitions may overlap with other voices.
Imitation is useful because it can help cultivate a sense of unity in a composition!
In Chapter 12, Ex. 5, we are asked to write a short piece for four instruments that utilizes four figurations derived from the pentatonic scale and has lots of imitation.