The spark plug hole is stripped.
Sometimes the hole can be saved, and sometimes it cannot. Methods for repairing the hole include chasing it with a tap, or using a HeliCoil kit. A HeliCoil is a coil-wire insert used to create new threads in the cylinder head. HeliCoils are not easy to use and their installation is best left to a professional. If the hole cannot be saved, a replacement cylinder head will need to be installed. I suggest calling an expert, such as one from YourMechanic, to inspect your head to see if it can be repaired.
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