Chris has set up a whole site dedicated to someone-else’s-server sites with links to resources and services (APIs), along with ideas of what you could build in this way.
Here’s one way to think about it: you can take your front-end skills and do things that typically only a back-end can do. You can write a JavaScript function that you run and receive a response from by hitting a URL. That’s sometimes also called Cloud Functions or Functions as a Service, which are perhaps better names, but just a part of the whole serverless thing.