Some modern aluminium rims designed for use with rim brakes have a groove, or a sunken dot in the braking surface. When the groove is gone the rims need to be replaced.
Inspect the bike wheel rim
If the braking surface is worn slightly concave it shows your bike wheel rims are getting near the end of their useful life. There may be some other signs of visible wear that indicate the bike wheel rim may need replacing.
For safety, the front bike wheel rim is more critical. If the rear bike wheel rim fails dramatically you stop. If the front fails you are more likely to crash.