A breach is an incident where an organization's internal system was accessed by an unauthorized third party, and potentially sensitive data was exposed. The type of data can vary, but can include customer data such as email addresses, passwords, payment information, or even your social security number. Criminals use this data for a variety of purposes, depending on the information they have obtained.
Why should you care?
If a criminal has your login credentials for a website that was breached, they can log into your account on that site and make purchases. If you use the same email address and password on other websites, the criminal could gain access to those accounts as well.