Fishnet is a type of knit fabric that features an open, diamond-shaped pattern. Fishnet is typically used in tights, stockings, or body stockings.
By its construction, fishnet visually enhances the muscles of the leg and thigh, which often leads to it being considered "sexy."
More About Fishnet
Fishnet stockings and tights are a common fetish. Fetishes for undergarments in general (including panties, stockings, garter belts, and pantyhose) are also rather common. Fishnet fetishes can be considered within that larger category.
A possible reason for enjoying fishnet in particular is the way it defines a person's muscles, and the way it reveals more skin than the usual nylon stockings. The fetish is based on the feminine associations with these pieces of fashion, as well as the visual and erotic effect of lingerie.