The adjective "imaginary" means the opposite of "real." Something that is imaginary exists only in the mind of the person "imagining" it. Related vocabulary words are the verb "imagine" which describes the act of creating an "image" or story entirely in one's mind and the noun "imagination" which describes the process of mental creation of a fantasy.
In the poem itself, the first person narrator is an oppressed person (presumably black, as can be deduced from the use of dialect, although there is no overt description) addressing a generic oppressor, and asserting that the trodden down and oppressed will "rise." The imaginary elements of the poem appear as metaphors and similes, usually ones of material wealth such as oil wells, gold, and diamonds. The real is expressed in such terms as sassiness, haughtiness, and sexiness.