A combolist is a text file that contains a consistent list of users and passwords. Combolists are designed to be machine-readable so that they can be used as input into applications that automate website or API authentication requests.
Credential stuffing attacks are a common example. A combolist might be used by a product like SentryMBA, an account checker, to automatically check which accounts are valid for a site. These accounts will be subjected to a full account takeover and, inevitably, fraud once they have been authenticated.