There are a lot of reasons why people might miss lectures. If you are missing a lot of lectures for medical or personal reasons, it’s best to contact your tutor, explain the situation and get their advice. If lecturers don’t know that you have a legitimate reason to be missing classes, they could assume that you are just not interested.
A lot of university lectures have a minimum attendance requirement. That might mean that if you have missed a certain percentage of lectures, your results could be affected, or you may not be allowed to sit the exams. This is why it’s very important to keep in contact with college staff if you are frequently missing lectures or if you are missing a large number of lectures/classes/tutorials.
If you are only missing one or two lectures/classes/tutorials per term, most lecturers will make allowances for this on the condition that you catch up on the work you’ve missed in your own time. However each course can have their own rules with regards minimum attendance so make sure you find out what the rules are for your classes.