A crimson force field was a fictitious defensive weapon named by Geordi La Forge and invented by Commander Riker. The force field effect was made by blowing hydrogen exhaust through the Bussard collectors, which created a harmless lightshow.
The crimson force field was used as a part of the deception designed to frighten the Pakleds into returning their hostage, Geordi La Forge, back to the USS Enterprise-D. The weapon could supposedly disarm the photon torpedoes of an attacking ship. In actuality, La Forge manually disabled the torpedoes while pretending to run tests to make sure they would work. (TNG: "Samaritan Snare")