A spool heel is a heel

Heel (shoe)

A heel is the projection at the back of a shoe which rests below the heel bone. The shoe heel is used to improve the balance of the shoe or for decorative purposes. Sometimes raised, the high heel is common to a form of shoe often worn by women but sometimes by men too. See also stiletto heel.-...

 that is wide at the top and bottom and narrower in the middle, so resembling a cotton reel. Spool heels were fashionable in Europe during the Baroque

Baroque

The Baroque is a period and the style that used exaggerated motion and clear, easily interpreted detail to produce drama, tension, exuberance, and grandeur in sculpture, painting, literature, dance, and music...

 and Rococo

Rococo

Rococo , also referred to as "Late Baroque", is an 18th-century style which developed as Baroque artists gave up their symmetry and became increasingly ornate, florid, and playful...

periods. Other periods of popularity include the 1860s and the 1950s. Spool heels are characterized by an hourglass shaped heel .i.e., thick at both ends and thin in the middle.