The magnetic containment field was a field that prevented the contact of antimatter with normal matter in a warp core. The magnetic containment field was produced by the magnetic constrictor coils in the core and concentrated the antimatter to the center of the reaction chamber. It was also produced by the power transfer conduits and near the antimatter pods. The field was also utilized to propel antimatter up to the reaction chamber.
If the field were to collapse or fall below 15% of its maximum integrity, the starship would be destroyed. Additionally, the magnetic field is vulnerable to quantum filaments, as they charge the vessel like a live electric wire, thus weakening the field by the inherent electromagnetic field that electricity produces. (TNG: "Disaster")