The course materials (e.g. video lectures, discussions, etc.) are usually free, but certain features like graded assignments, certificates, and mentorship may be paid features. Course and/or certificate costs vary for each MOOC provider. Courses may be available either on a subscription basis or for individual purchase.
Free access may be called audit mode. If you are not clear on how how to sign up for Coursera courses for free, here is a guide from Class Central: A Guide on How to Sign up for Coursera Courses for Free Other course providers may have a similar process for free access.
Also see Will I receive a certificate for completing a MOOC? and Are Coursera courses free? An investigation by Class Central discovered 1150+ Coursera courses that are still completely free, without a certificate.
A small number of courses are not free. These are indicated by a small green $ symbol on Class Central.
If you want a certificate but cannot afford the cost, some providers offer financial aid to needy applicants. See Can I get financial aid for MOOCs?