Good question! When you "make out a check", it means you are writing a check or filling it out. The person/business whom the check should be paid to is who you make the check out to. In this case, the person was angry and instead of filling the check out correctly, they were saying they were going to write "garbage" in who the check should be paid to. They weren't seriously going to do this though and were being sarcastic because they have a negative opinion of who they are supposed to be paying. I hope this helps! Also, if you're interested in conversational English lessons, I'm a qualified and experienced English teacher and would be happy to work with you! :-) You can find me here: https://teach.italki.com/teacher/5129645