Petrification is a state of vulnerability on Trollhunters. It can be caused for three reasons: a troll being exposed to sunlight, someone being cut by Creeper Sun, or being stabbed by a magical object like the Swords of Eclipse and Daylight.
The troll turns to ordinary grey stone, a statue of himself/herself. This statue is very brittle and will break on application of any slight force.
If the troll was petrified using Creeper's Sun, then the effects can be reversed by using the Creeper's Sun antidote.