Shift Out (SO) and Shift In (SI) are ASCII control characters 14 and 15, respectively (0xE and 0xF).  The original meaning of those characters was to switch to a different character set and back.  This was used, for instance, in the Russian character set known as KOI-7, where SO starts printing Russian letters, and SI starts printing Latin letters again.

On many printers, however, SO starts printing double-width characters, and SI enters "condensed" mode (narrow characters).