Sometimes an "audit" is just an informal agreement between the professor and a student to sit in the classroom during lectures and take notes and ask questions but not take quizzes or exams nor do any homework or projects or get any marks or feedback. I certainly audited several classes during my undergrad in the mid to late 90s this way. I also stopped showing up to those lectures as soon as it became too time consuming or the prep work was too much to keep up with.
Some universities may allow for a paid audit where less tuition is charged and everything is normal but no credit is given in the end. The upside for the student being that there's no pressure to pass and no ultimate grade assigned in the end on the transcript just an "audit" mark or nothing. I don't think I've ever seen one of these transcripts, but I don't review very many undergrad transcripts, ever.
I just found the official audit form for my undergrad university, and it didn't even allow for a transcript record and required full tuition for the course, so YMMV. So in the end, no, I agree with others, unless your university is weird or you go off the books and audit the course without telling your university registrar or paying (and don't get caught!!), your grad school application won't be affected. If you can, it's probably best to do the honest thing and pay for an official audit, but you have to choose your own ethics and settle that with the professor who is teaching. They will notice if there's an extra, unaccounted for student in the lectures!