in the link of the challenge, they essentially explain what ‘serve html’ means
Send the /views/index.html file as a response to GET requests to the / path. If you view your live app, you should see a big HTML heading (and a form that we will use later…), with no style applied.
When a server responds to a GET request by ‘serving’ an html file, it is doing what you expect everyday when you open up your browser and click a link.
You are essentially requesting a serve r somewhere to give you the contents of a certain html file (at a certain URL) and that is what it means “to serve the html file”.