Almost everybody has a web presence of some kind, and if you don't, you should consider it. All of us have interests or hobbies that other people don't. You probably even read a few blogs related to things that you're interested in. The blogs that you're reading probably make a little money every time you visit, because that's the magic of Google AdSense.
If you have a website that has a solid, regular viewership, advertisers want to put their products on it. That's where AdSense comes in. It partners with advertisers and matches them with websites based on their interest or specialty area. If you have a blog about fishing, for example, they'll display fishing adverts on your site. If you have a blog about travel, expect to see travel agency adverts appear. It's simple stuff, but it's a way of getting a passive income from your hobby site that grows as it becomes more popular.