Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks, and rays, as well as primitive fish such as lampreys.
Regarding this, what do pharyngeal gill slits become in humans?
Pharyngeal slits resembling gill slits are transiently present during the embryonic stages of tetrapod development. However, it is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates.
What vestigial structures do embryos have?
Charles Darwin listed a number of putative human vestigial features, which he termed rudimentary, in The Descent of Man (1890). These included the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the eye.