A modified car is one which has had changes since it was produced by the manufacturer. This includes extras that have been added such as alloy wheels, exhaust changes, or tinted windows. If you're buying a second-hand car, you should check if any changes have been made.
Modifications can impact the value of your car and your likelihood of having an accident, so insurers will want to know about any changes you may have made to the vehicle since it was manufactured.