White polyester water-blocking tape is a type of tape used in cable applications to prevent water from penetrating into the cable. It is typically made of polyester fabric and has a water-blocking material such as a superabsorbent polymer (SAP) embedded in it. The tape is applied longitudinally over the cable core, and when it comes into contact with water, the SAP swells and forms a gel-like substance that fills any voids and prevents water from entering the cable. This helps to protect the cable from water damage and ensures its proper functioning.