Term men will use for a woman (while the term could be used in reverse, its rarely seen), usually a very attractive woman, who a man has befriended. While the friendship itself could turn into something more, one of the fringe benefits of being her friend is being introduced to her friends, (who are also equally, even exceedingly, attractive) with a strong letter of recommendation, from the original girl 'friend.'
The dilemma for the man becomes - does he work to turn the friendship into something more, or work his way into the network? If he works to build something more out of the friendship, he automatically becomes 'off limits' to all of the first girl's friends, regardless of the outcome with the first girl.
The saltshaker is the girl 'friend.' If the man opts to date his friend (saltshaker), all the other spices are suddenly taken off the table, never to be returned.