The phrase typically refers to a combolist — a text file containing 400,000 sets of usernames and passwords (or email/password combinations) used for unauthorized account access . In the context of gaming and cybersecurity:

Specifies the game these credentials are intended for (Riot Games accounts).

Files like these are often distributed in "cracking" or "leaking" forums. Using or downloading such files carries significant risks:

If you are concerned about your own account security, it is highly recommended to enable through your Riot Games account settings and use a unique password that hasn't been used on other sites.