The submarine hatch forms a hermetic seal.

Hermetic means complete and airtight, insulated or protected from outside influences. As in (of a seal or closure)

To be hermetic means to be completely insulated and air tight. It could be applied to a seal to say it is airtight or in a human being hermetic would mean not being influenced from the outside.

The informecial claimed that the new containers used modern technology to guarantee a hermetic seal that would keep food fresh for months

The hermetic refers to being air tight. The term commonly used is hermetically sealed.

An example of a hermetic container would we a glass jar, or any container with an air tight seal.