The website is free. The courses include some interactions with the teachers but as long as you register during the time they accept registration, the ressource will be accessible forever.
Many popular course are re-run each year or so, evolving and improving. So, you can try to better understand what you didn’t get from the previous edition or simply register to the course you miss last time.

The level of the program greatly vary from one MOOC to another. Some advertised to be for 18-year old young student. Some help me complete the program of my master degree.

There is also french course, especially for people who need the vocabular to start living in France or work in French.