Acrostic poems are poems in which you use first letters or the letters of a word in a sentence or phrase to develop codes so that you can remember the poem. For example let's take a poem-
Oh look at the moon,
He is shining up there,
Oh mother he looks
Like a lamp in the air.
This poem can be remembered by letters if it forms a word such as Lump as look, up, Mother, lamp etc.