A grayscale is a one-dimensional color scale: every tone in it is fully described by a single number, its lightness, running from 0% (black) to 100% (white). In an 8-bit digital image that range is divided into 256 steps, which is where the familiar '256 shades of gray' figure comes from.
In fabric, grays also carry undertones. Warm grays contain a little brown or red and sit naturally beside cream, tan, and brown leather; cool grays contain a little blue and pair cleanly with white, black, and silver hardware. Checking garments in daylight is the simplest way to see which family a gray belongs to before combining pieces.