An abecedarius is a poem in which each line or stanza begins with a successive letter of the alphabet. Though in modern societies abecedarii are usually thought of as childish, there is a long history of quite serious abecedarian poetry, including several biblical Psalms (in Hebrew). There are also examples in Classical and Hellenistic Greek, Medieval Latin (St. Augustine), Byzantine Greek, and Middle English (Chaucer).
To conceive of how this type of form had significance, we can liken it to numerology, in which numbers are given mystical meaning. Historically, alphabets have sometimes been given similar significance, which explains why abecedarii are often religious in nature.
One famous example of an abecedarius is Chaucer’s “An ABC,” which is made of 26 eight-line stanzas, each beginning with a successive letter of the English alphabet.