In SEO ranking - I've read one GOOD backlink is worth 100 bad backlinks. How do I know what is a good or bad backlink?

Hi All! Beginner here, thanks for any help!

I have read a few times that backlinks are KEY in SEO but they can't just be trash backlinks from anywhere. How does one actually know if a backlink is good or bad, is there some sort of system for measuring? I could get my site on yelp, yellowpages, a bunch of random directories but how do I know if they are good? If I want to reach out to other blogs, how could I measure if they are good investments?