A company will give you a sample to try for free (we will use "Face Wash" as an example). You pay $1.95 to have the face wash shipped to you, and if you love it, the company will ship you the face wash every 30 days and charge you $19.95.
You can go to their website, send an email, or give them a call and tell them that it's not something you want to continue to receive. As long as you have done so within the time frame they gave you for the trial offer, you will not have to pay another dime.
So, you have paid the $1.95 for shipping of the product and the offer is a thirty-day trial. The company pays $20.00 for the offer and if you don't like the product, you can return it before the 30 days - but you keep the $20.00. You had the chance to try the face wash and profited $18.05 for doing so. I am sure you are already beginning to understand how doing 5-6 trial offers per month could easily bring in an extra $100.00.