When do I use d2 in Ford Taurus 1994?

I am driving Ford Taurus GL (1994) on D1. It has D2 too. I have never used it so far. When do I use it?

Also, I can't figur out why its Antilock light it on. The car is running good, I havent noticed any problem with it, but this light is worrying me.

Thank you.