We hear this term use to describe something that has an airtight seal, such as the answer envelopes that Ed MacMahon would give to the Great Karnak (Johnny Carson) to hold up to his head and divine the answers inside.
The expression "hermetically sealed" comes from about the third century AD. Hermes Trismegistus, meaning Hermes thrice the greatest (three times greater than the original Greek god Hermes), was said to have been the Egyptian god of wisdom, Thoth. Hermes supposedly dictated twelve Hermetic Books to mystic philosophers, containing secrets for alchemists. One of these secrets was how to seal a vessel so no air could get in or out. All he had described was a simple, hand-operated vacuum pump.
Today, there is nothing magical or special about containers that are "hermetically sealed." It is simply a fancy way of saying airtight, which honors the ancient god Hermes.