They connect to have an airtight sealed plug, and are made out of plastic sealed plugging. Another type of waterproof connectors is the plastic sealed pluggable type, and is the other type of electrical connector.
Waterproof connectors are made of water resistant seals, which are able to withstand water from a variety of seals. Some materials also include waterproof seals, for instance, aluminum, PVC, and Types, aluminum waterproof connectors, and many more.