Feldspars are aluminosilicate minerals that contain sodium, potassium or calcium. In ceramics, sodium and potassium feldspars act as fluxes: they melt during firing and form a glassy phase that fills the pores and binds the other particles.
The more feldspar melts, the denser the body becomes. A porcelain recipe relies on it to reach a water absorption close to zero at a firing temperature the kiln can sustain.