It's hard to pinpoint. It's everything that's not your genes, which could mean where you live, what you eat, chemicals you've come into contact with, and more.
It could take years for the effects from something in your environment to show up. So far, doctors have a lot of clues but no smoking gun. For example, you could have people who live or work near chemicals linked to Parkinson's, but many of them don't get it.