Combos
A combo "combos" are a user:pass login which can either be real or fake.
These can be obtained several ways for example: Scraping, SQLi (SQL injection, a form of hacking into a website to then dump the real user:pass database) and using public dumps of combos
You do NOT look for combos aimed at specific sites, any user:pass could work on any site, you could use SQLi on a porn site and get hits with runescape, two totally unrelated. Most new people really don't understand this