Steam combo list is a text-based compilation of username and password pairs used by cyber attackers to gain unauthorized access to Steam accounts. These lists are typically formatted as username:password email:password
End of report.
Happy hunting, and may your frame rates be high and your prices be low!
Websites like Have I Been Pwned allow you to check if your email address appears in known data breaches. If your email pops up, it means your credentials are likely sitting in a combo list somewhere.