It is the six-digit hexadecimal code for the color — 0055BF for Blue, for example. The swatch beside each row is rendered from that value, so it is the quickest way to compare two shades on screen.
It marks whether a color is one of LEGO's see-through plastics — the material used for windows, windscreens and light covers — rather than a solid color. It is the fastest way to tell a solid shade from its transparent counterpart.
LEGO has produced many close shades over the years, and catalogs do not always name them the same way. When two names look interchangeable, compare their RGB values.
In the parts catalog. A part's page lists every color it has been produced in, each with its own BrickLink, BrickOwl and Pick a Brick price.