A transcript with many W s is not good. If you wished to transfer to another school and have many W s on your transcript, you may find it difficult to be accepted into their programs. In general, classes in the U.S. are more regulated and make more constant demands on student time throughout the year than is the case in other educational systems. Professors cannot grade solely on attendance, but most expect people to attend all classes. Some will assign credit for class participation, and it is difficult to participate without being there.
Participation usually means taking part in class discussions or class projects.
Large classes are usually lecture, but in smaller classes many professors expect student to express opinions or answer questions about the readings. You are expected to keep current with the readings. Syllabi should have some form of course schedule to help you know what will be the topic of each class. It is important to be sensitive to the personality of the professor. Some may encourage discussion, but do not really want to have students disagree with them.