These are spring feet that fit on the bottom of a RepRap machine to damp the transmission of vibrations from the machine to whatever surface it is resting on.
If that surface is resonant, a RepRap machine without these feet can make quite a bit of noise. These reduce that noise significantly.
Print two of the feet with projections on the bottom, and two of the ones without.
The ones with the projections fit under the higher bottom threaded-rod clamps on one diagonal, the no-projection ones fit under the other diagonal.
These are designed to be printed on their side, so that they look like a letter Z when viewed from above.