Those are good ethernet couplers, made of strong plastic. They are a good solution for patch panel connections of a rack. The price is bit high, but the quality seems to be very good!
EDIT: THERE IS A BAD USAGE OF PLASTIC ENVELOPS. THE KEYSTONES CAME TO ME IN ONE PLASTIC ENVELOP, WHICH CONTAINS 10 DIFFERENT PLASTIC ENVELOPS! IT'S A WASTE...
very good