PCI compliance is a set of rules and regulations that apply to any business, of any size, that accepts credit card data. For a business to be compliant requires a specific set of safety protocols designed to protect both the business and its customers from fraudulent activity.
In essence, the major card brands have created a system to verify that you’re meeting pre-set security standards to be compliant. These standards are updated regularly, to help merchants mitigate potentially unsafe card practices. By becoming compliant, you’re ensuring that your business is staying as protected (as much as possible) from fraud, and accepting credit cards in the manner that Visa/MC/Discover/Amex deem safe and appropriate.