Oh, right. Force of friction equals coefficient of friction times force normal.
Force normal is the amount that the ground pushes up on you to keep you from falling through it. Basically your weight.
Coefficient of friction is dependent upon the two surfaces in contact. Although rubber tends to have a high coefficient, ice is very low. Rubber on ice is somewhere from 0.1 to 0.4. Thus, there is little friction and you tend to slide.