>> Sadly, it's not anywhere on the page text but the <title> makes it clear: “for learners and educators”
It is right on the front page: "Thimble is an online code editor that makes it easy to create and publish your own web pages while learning HTML, CSS & JavaScript."
The text of the <title> element is not present anywhere on the page, even though there's plenty of room in the logo at the top element to expand it.
The “while learning” bit is a nice nod in that direction but it's in de-emphasized text at the end of a block (the only other text that small is the login link in the upper corner). Given the number of people who didn't get why the project exists at first, I would suggest that it should be more prominent.
This could be as simple as changing the highlighted text “Write the Web On the Web” title to “Learn to Write the Web On the Web” so it doesn't give the impression that this is intended to be a general-purpose editor.
It is right on the front page: "Thimble is an online code editor that makes it easy to create and publish your own web pages while learning HTML, CSS & JavaScript."