The ogham alphabet stores a letter as a position: which family, and how many strokes.
How the letter is formed
Saille is the fourth letter of the first family, Aicme Beithe, and is cut as four strokes on that family's side of the line. On a stone the line is the arris and the strokes run out from it; in the manuscripts the line is horizontal and the first family sits below it.
Why four strokes is the practical limit minus one
Five is the maximum count in any family, and it is not an arbitrary ceiling. A reader counts strokes by eye, and beyond about five the count stops being reliable at a glance on a weathered stone. Four families of five gives twenty letters, which is close to what the language of the inscriptions actually needed. When the manuscript tradition wanted more characters it did not extend the count past five; it invented a fifth family of composite shapes instead, the forfeda, which look nothing like the first twenty and were never in ordinary epigraphic use.
The sound
Saille carries the value S, one of the commonest consonants in the personal names that make up almost the whole surviving corpus. Those inscriptions were cut in Ireland and in Irish-settled parts of Wales, Scotland and the Isle of Man, and are dated on linguistic grounds to roughly the fourth to sixth centuries.
The willow attached to this letter comes from the later manuscript glosses and is treated separately here.