א (Aleph, the first letter of the Hebrew alphabet) stands for the famous Codex Sinaiticus (sometimes designated by the letter S), one of the two oldest copies (apart from the papyri) of the Greek Bible.
This page, (quire 36, folio 6r,) is from the book of Esther, Esth.1:15-2:14.
The book of Esther is a short historical novel, set in Susa, in the reign of Artaxerxes, king of Persia. In a banquet after his marriage, Artaxerxes' new queen, Astin, disobey's the kings request to appear before the guests. In chapter 2, the king is captivated by the beauty of Esther, who becomes his new queen.
The passages on this folio can be viewed in context HERE and HERE.
More about Codex Sinaiticus HERE, especially (from the Sinaiticus web-site!) HERE.