means A keyboard shortcut that selects all content in the current document or field at once, letting you act on everything without clicking around like a medieval peasant.

from Born in the 1980s when computer interfaces realized humans were too lazy to manually highlight text. The A likely stands for All, though early systems just needed a memorable key that wouldn't conflict with anything else. Became universal because it's faster than any mouse alternative and works the same way whether you're in Notepad or a billion-dollar spreadsheet.