A primer of the alphabet — the letters gathered first, so everything spelled later can be read. An abecedary holds no sentences; it holds what sentences are made of.
Medieval Latin abecedarium, a noun built directly from the spoken names of the first four letters A-B-C-D plus the suffix -arium (a thing or place for); a primer named quite literally after the alphabet's own beginning.
The letters gathered before any sentence is built from them, so what comes later can be read at all.