A metaphasic shield is a modified defensive shield that allows a starship to operate in close proximity to a star for an extended period of time.
What actually occurs is unclear, but the name suggests that a metaphasic shield differs from the standard frequency pattern of Star Trek shields. Normally, the shield oscillates at a specific rate, creating peaks and troughs in the protection afforded by the shield. The hypothesis for metaphasic shields is that there are multiple waves in the shield, with their peaks and troughs offset, leaving no low points in the defense. The downside of a metaphasic shield is that weapons like phasers cannot be synchronized to "fire through the gaps", so a ship using metaphasic shields cannot fire weapons.