A car that has been in an accident has a reduced market value than a comparable vehicle in pre-crash condition, depending on its age and condition. Potential purchasers will not pay as much for a car that has been damaged previously, even if the car repairs are of the best quality.
The gap between the pre-accident car's market value and the post-repair car's market value is referred to as reduced value.
The information in this answer is general in nature and is not legal or insurance advice for your specific situation. Policy language varies carrier by carrier; your declarations page is the final word. If you're unsure, call your agent before you need to file.