Look closely at a used brake rotor and you may see tiny lines across the friction surface. Some fine cracking is normal heat checking; deeper cracks are a warning. Here is what heat cracks on a brake rotor mean.
Heat checking vs real cracks
Fine, shallow surface lines - called heat checking - come from repeated heating and cooling and are usually cosmetic on an otherwise good rotor. Deep cracks you can catch a fingernail in, or cracks reaching the edge, mean the metal is failing and the rotor must be replaced.
What causes them
Heat cracks form when a rotor sees extreme, repeated heat - hard mountain descents, heavy towing, or an aggressive pad - especially if the rotor was overheated quickly; see why brakes get hot. Cheap rotors with poor metallurgy crack sooner. Hard spots in the metal can also concentrate stress.
Why deep cracks are dangerous
A deeply cracked rotor can lose strength and, in the worst case, fail. Deep cracks also cause vibration and uneven pad contact, similar to a warped rotor; see warped rotor causes. You cannot machine deep cracks out - the rotor needs replacing; see resurface vs replace.
Prevent and fix
Avoid riding the brakes on long descents, let them cool, and choose quality rotors with good heat capacity. Replace cracked rotors in axle pairs and bed in fresh pads with new hardware.