The weight of a font specifies its boldness. Arranged from lightest to heaviest (more or less):
hairline, extra light, light book, regular, medium demibold, semibold, bold, extra bold, heavy black, ultra, poster
Here is the table, from the file weight.map. It is organized alphabetically by abbreviation. Each line consists of an abbreviation and any parts of a PostScript ‘FontName’ which use that abbreviation.
a Thin Hairline b Bold c Black d Demi h Heavy Heavyface j ExtraLight k Book l Light m Medium p Poster r Regular Roman s Semibold u Ultra UltraBlack x ExtraBold ExtraBlack