Students are expected to attend every class. Missing one or two classes during the semester due to important reasons (e.g., travel, sickness) is fine. However, if you expect to miss class more than twice during the semester, please notify the instructor prior to missing class.
Of course, attendance factors into your participation grade.
If you must miss a class, make sure to get lecture notes from one of your peers. Hint: class discussions are an excellent source of exam questions.