Yes. In any lesson, press Notes in the toolbar and type. Your notes save as you type, and a dot on the button shows which lessons already have a note.
To see them all, click your name at the top of the page and choose My notes. They are listed by course in course order, each with a link back to its lesson, and you can download a course’s notes as a PDF.
Notes are for your own words: a key point, a question for your tutor or an idea to try. Text copied from the lesson can’t be pasted in. Notes aren’t available on assessments.