You find a free trial of a product or service and decide to give it a try. You may have to pay for the shipping or enter payment information. Once you have entered your card details the free trial starts or the trial product is sent. There may not be any mention of further charges when you place the order or start the trial. You receive the product within a few weeks but within a month you may notice a further payment on your account as this trial auto-renews. These companies rarely send reminders when these subscriptions are about to renew.
Auto-renewals, themselves, are not prohibited within consumer law but the company must inform you within their terms that the subscription will renew and provide clear instructions on how to cancel this, so check the terms carefully before signing up to a free trial of a product or service.