CVS is a drug store - like Walgreens or Rite Aid.
They have a shopper reward program card called the Extra Care Program. When you buy certain items from the ads each week, you get back something called Extra Care Bucks. They are like gift certificates that you can use on subsequent purchases (there is an expiration date on them). You can generally combine these ECB with coupons from the Sunday paper or the internet and get items free - like toothpaste, shampoo, razors, etc.
When people say they are "CVSing" it usually means they use their ECB and coupons from one week to buy items that produce more ECB the next week, thus having to pay nothing or very little "real" money. For around $20 startup with your first purchase to get you going, you can "roll" these ECB each week to get free stuff and not have to pay anything (except tax) from then on.
Read the weekly CVS thread where people write down what they purchased and you can get a hang of it after a couple weeks. Just copy what people do and you don't even have to figure it out on your own!! Its really easy to get free toiletries. You just have to be disciplined to not be tempted by other stuff (like grab a bottle of coke or a candy bar or kids toys) or you can diminish your ECB.
Maggie