Back in the 80s, most home computers used to boot into a dialect of BASIC. This made it very obvious how to start to learn to program - just type things in and try things out to see what works.
The code editor might have syntax highlighting, intellisense, a built-in debugger - whatever can be provided to make programs as easy as possible to develop.
Once you've written some code you can save it on the website and access it from anywhere. You can also share it with friends. If one person defines an object someone else can use that object in their programs. In this way, a rich ecosystem of scripts can develop.