The maximum time a frame can take. Physics and other fixed frame rate updates (like MonoBehaviour's FixedUpdate) will be performed only for this duration of time per frame.
Use this function to avoid low frame rate spikes occurring due to combination of GarbageCollector and expensive physics or other fixed frame rate updates.
If frame will take longer than specified value, then less physics and other fixed frame rate updates will be performed. That will temporarily slow down the gameplay, but will allow frame rate to catchup avoiding nasty stuttering.
It is advisable to keep maximum time for frame between 1/10th and 1/3th of a second.