Coursera primarily offers two different kinds of certificates: a course certificate and a Specialization certificate. Specializations are a micro-credential offered by Coursera and consist of a sequence of courses. You must pay if you want a certificate, unless you successfully apply for Coursera financial aid.
You can then work through the course materials and complete assessment items. Earn a certificate by gaining the required number of marks for the course.
Required pass marks can vary from course to course. Most courses give you multiple chances to re-do quizzes and exams. For courses with peer assessments, you may be able to re-submit an assignment if your first attempt was unsuccessful, but you will need to check this within the individual course.
To earn a Specialization certificate, you must earn individual course certificates for all the courses that are part of the Specialization.