If your brake light is coming on and off while you're driving, there are a few possible explanations. It could be a simple issue like a loose wire or blown fuse, or it could be a sign of a more serious problem with your brakes.
If the light is coming on and off intermittently, the most likely explanation is a loose wire or blown fuse. Check your vehicle's fuse box and replace any blown fuses. If that doesn't fix the problem, then it's time to check the wiring. Inspect the wiring going to your brake light to make sure it's not loose, damaged, or corroded.
If the light is staying on constantly, it's a good indication that there's a problem with your brakes. The most common cause of a brake light staying on is low brake fluid. Check your brake fluid level and top it off if necessary. If the fluid level is fine, then the problem could be a faulty brake light switch or a problem with the brake hydraulics. In either case, it's best to have a professional mechanic take a look.
Hopefully this gives you a better idea of why your brake light might be coming on and off while driving. If you're still having trouble, don't hesitate to take your vehicle to a mechanic for further diagnosis.