What causes the ABS light to come on? on 1994 Mazda B2300

The ABS light comes on pretty often. What is wrong that causes this to happen?
Is this really a serious problem?

Asked by for the 1994 Mazda B2300
When your ABS Light is on, you will not have your anti-lock brake function, which is very important is slippery road conditions or during a panic stop.
The ABS light can come on for many reasons, so it's best to get it looked at to determine the cause. A shop would generally charge you a diagnostic fee to look into this problem, and it will be approx an hour of diagnosis time.
2 answers
I have same 1994 MAZDA B2300 and ABS light just start turn ON. My truck does not have any ABS system at all. It is simple 2 wheel drive, 5 speed manual transmission.
If it show on dash board ABS it does not means that vehicle actually has ABS system at all.
I will just ignore it. Brake works good any way.