It started off as a way for people to get info on cars via fax machine. You ask or enter the vehicle VIN and you get a report. If it was in a crash, in a flood, see if mileage matches actual odometer. It's all web-based now.
Vehicle history reports. It's mostly to see if a used vehicle was in an accident prior to purchase though it will sometimes also tell you if the car had a history of maintenance (typically this indicates a car was serviced at the dealer).
https://en.wikipedia.org/wiki/Carfax,_Inc.