At the end of a hand, all the cards get rolled over and cards speak. In hold'em, a rock turns his hand over and proudly declares "three queens." If you then study his cards, and peek at your cards, and turn over one card, and then look back at his cards, and then peek back at your remaining card, and then slowly roll it over and declare "you better get that fixed, I've got a straight to the ace" you have slow rolled the rock. In other words, you have rubbed salt in his wounds.