The spade symbol has a large assortment of meanings from military units, gang signs, to ancient Greece, however the black spade with a white "Q" in the center is specifically referring to a "Queen of Spades".
Traditionally the QOS dynamic is a hot-wife that exclusively swings with black men under full support of the husband.
A woman wearing such a symbol is advertising to black men she is publicly approachable, to be flirted with, and possibly available for sex regardless if her husband is around or not.
A single woman would not be considered a true QOS.