CodePen is an online code editor for front-end development. You build things (we call them Pens) with HTML, CSS, and JavaScript, and the languages and libraries that go along with those. Any Pen can be embedded on another site, making them useful for blog posts, documentation, forums, etc.
This block makes it easy to embed them. While oEmbed (just paste in a Pen URL!) also works for CodePen embeds in the block editor, this plugin allows for much more control, like setting the the height, theme, and default tabs.