Keeper liability is the rule that makes the registered keeper of a vehicle responsible for certain charges when the driver is not identified. It lets parking operators and charging schemes pursue the keeper for an unpaid charge.
In one line: Keeper liability makes a vehicle's registered keeper responsible for an unpaid charge when the driver is not named.
How keeper liability works
For private parking, keeper liability comes from the Protection of Freedoms Act 2012, which lets an operator recover an unpaid charge from the registered keeper if strict notice rules are followed. Similar principles apply to road charging schemes enforced by camera.
For example, if a car incurs a 100 GBP private parking charge and the driver is never identified, the operator can transfer liability to the keeper named on the V5C, provided the notice was served correctly and on time.
If the operator fails to meet the procedural and timing requirements, keeper liability does not transfer, and the keeper may have grounds to challenge the charge.
Keeper liability vs driver liability
Driver liability rests on the person actually driving, which matters for criminal offences like speeding where the driver must be identified. Keeper liability shifts responsibility for certain civil charges to the registered keeper when no driver is named.
This is why a NIP for speeding asks who was driving, while a private parking charge can fall on the keeper by default if the rules are met.
Primary source: Protection of Freedoms Act 2012 (legislation.gov.uk)