If you don't pay, you still get the exact same learning materials, but you don't get any interaction with instructors or any grades on your assignments.
Coursera is different from many other online learning sites because it hosts real courses from prestigious universities and makes many of them available for free. In other words, you can get all the lectures (recorded to video) and reading materials from a class at, say, Yale University without paying anything at all. Coursera has other kinds of classes, too, but the university material is really what sets it apart. For some courses, there are options to pay for it and earn a professional certificate, bachelor's degree, or master's degree. If you don't pay, you still get the exact same learning materials, but you don't get any interaction with instructors or any grades on your assignments.