Plato regards imitation as a negative activity, one with little value and one that is mainly a waste of time. He thinks that literature, almost by definition, is imitative in this negative sense. Poets have no real intellectual access to the rational forms and ideas that Plato values. (Philosophers do.) Poets can only imitate imitations of those ideas. The idea "chair," for instance, is accessible by the use of the logical intellect. The actual chair is a particular imitation of that general idea; a poem or painting about a particular chair is an imitation of an imitation and is thus doubly removed from truth. Creating imitations of imitations is a generally worthless pursuit. For more discussion of this point, see Plato's Ion.
In contrast, Aristotle values imitation as an intellectual operation in its own right. He believes that all humans learn by imitating and that imitation is a natural human instinct that reflects a natural, innate human desire for knowledge. For fuller discussion of this point, see Aristotle's Poetics.
For detailed, point-by-point comparisons and contrasts of the ideas of these two thinkers (and of about fifteen other ways of looking at literature), see my book Close Readings.