The style of shape notes used in the Old-Line Primitive Baptist Hymn and Tune Book was invented by Jesse B. Aiken in around 1846 as an extension of the earlier 4-shape system used in books such as the Sacred Harp and Southern Harmony. Though they have largely ceased to be used in the broader world, our brethren have used them for years to learn unfamiliar tunes without mastering all the complexities of classical musical notation.
Each note shape corresponds with a note of the musical scale, e.g. do, re, mi, fa, sol, la, ti, do:
As an example, here is the tune Old Hundred rendered in shape notes with the note names written below: