A combolist is a text file containing a list of username (or email) and password pairs. These lists are usually compiled from multiple data breaches across various websites.
Use reputable services like Have I Been Pwned to see if your email address has been included in known data breaches.
Generate unique, complex passwords for every single account so that one breach doesn't compromise your entire digital life.
Cybercriminals don't usually log into these accounts manually. Instead, they use bots to perform attacks.