Sometimes you just don't have time to launch your code editor and a web server, not to mention time to host on external servers. Or you might want to make a quick presentation, fix an idea, test an algorithm or an external library, try a new design, publish an article.
A notebook consists of cells. Each cell is a snippet of JavaScript, CoffeeScript for the nostalgic and sensitive, HTML or Markdown code. Much like Jupyter notebooks. Unlike Jupyter, you can hide the code or results of its execution. You can annotate cells, which is handy when the cell is hidden.