What does "engine in fail safe mode" mean? on 2000 Jaguar S-Type

What does "engine in fail safe mode" mean?

Asked by for the 2000 Jaguar S-Type
That happen to me once while I was driving. When your car is in Fail safe mode that means there may be a problem with your engine and for safety your car will lower its performance to reduce any further damage to the engine. Sometimes it it could be your sensors messing up happens when you drive to much on a bumpy road or pot hole or something in your car is leaking and slowly losing liquid level like my coolant tank!!!
how critical is this and how expense to repair if you know, please.
1 more answer
keeps the car from overheating