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Overall view of the Skylab shoe and triangular floor grid display.

Sections of Skylab's flooring were composed of a triangular grid and the astronauts' shoes had a triangular cleat which would protrude through the floor and twist to lock. Once properly anchored, the astronaut could then exert force on objects without that pesky "equal but opposite reaction" from hindering him.

Refer to Skylab, Our First Space Station for information and a photo of the Skylab shoe and grid system. Also see the Smithsonian's page on a pair of flight shoes (scroll down to the tenth photo for a view of the bottom of the shoe).

Here's a picture of the flooring in one part of Skylab (which, coincidentally, also shows the Lower Body Negative Pressure experiment.

The sign below the display reads

This is a piece of floor from Skylab with a pair of astronaut's work shoes. Since an astronaut floats around in the weightlessness of space, it is necessary for him to twist-lock his shoes to the floor before he can do any physical work.