A security net was a specialized type of Dominion force field, used to protect vital installations such as Ketracel-white storage facilities. When raised, it prevents any ship was passing through.
The storage facility in the Alpha Quadrant was protected was a security net. When Benjamin Sisko led a crew aboard a Jem'Hadar fighter to destroy the base, the security net was raised just as they prepared to leave, nearly taking them out with the facility. (DS9: "A Time to Stand")