a yellow light "ABS" turn on on my 2002 F-150, what does tnis mean? how can I fix it?

ABS is the anti lock brake system. If they did an alignment chances are they damaged something or did not reset a sensor properly. I would take it back and insist they look at it again. Any decent shop should be able to diagnose an ABS light. If this happened at my shop my foreman would not give me any more jobs until I figured it out. They should diagnose and fix it for free. The chances of it being a coincidence are very slim. The only way you could do something on your own is if you have a scanner and can get the code that is causing the light.

In some cases they could have knocked one of the wheel speed sensors loose or the wiring when they made the adjustment. I'd take it back to who ever did the alignment and find out.

But as the other poster stated, there is a code you can pull which would save a whole lot of hassle.

In some cases it could be just a coincidence and you need to check the resistance on each wheel sensor.

4x4 = 300 to 500 ohms // 2 wheel drive should be 2000-4000 ohms.

Or just take it to a shop that does brake work and ask them to read the codes only and tell you what is wrong.

ABS Anti-Lock-Brakes go see If theres a code.