It stands for »Standard«. It is used for OpenType fonts that only have a standard set of glyphs, like Mac Roman, Win 1252 and so on.

Ralf

Generally, Adobe's "Std" fonts lack the extended language support that you find in their "Pro" fonts.

Nevertheless, they are not necessarily limited to a small glyph set. Depending on the font, they may have many glyphs more than you'd expect: for example, Poetica is a "Std" face, but has a great number of alternate glyphs, Kepler is Std, but has ornaments, small caps and swashes.