Forms of discourse are the main ways a poem or passage can present its subject: description paints a scene or object, narration tells what happens, exposition explains an idea or situation, and persuasion tries to convince the reader of something. Poems often blend more than one form, but one usually leads.
Example
The porch glowed gold at dusk, the lantern's small light warm. (description)
I shut the gate, crossed the yard, and found the note. (narration)
The creek floods after heavy rain because the banks cannot hold the water. (exposition)
Leave the city now and breathe where the pines can teach you silence. (persuasion)