It's very important. After you receive your first notification of selection results, you must reply to your offer – and answer yes – to the courses and programmes that you'd like to keep your place in. Be sure to do this before the deadline.
By answering yes, you keep your place for the second selection. If you don't answer yes, this will indicate that you do not wish to accept your place in the course and it will be offered to someone else.
The same applies to courses and programmes you've been placed on a waiting list (reserve) for. If you wish to be considered in the second selection, you must answer 'Yes' to reserve places. Otherwise, your waiting list place will be deleted.
You'll receive a second notification of selection results (admission results) with the results of the second selection. This one you won't need to reply to.
If you applied for the First round
Applicants receive only one notification of selection results - admissions results - when they've applied for the first round. You do not need to reply to your offer in order to keep your place. However, it's important that you decline any places you don't wish to keep. This will let the university know you won't be attending, and also free the place for someone on the waiting list. Please note that the university may contact you with a deadline for your reply - be sure to follow their instructions.
If you wish to withdraw from a course you've already started attending - even if it's just the first meeting - you should contact the university and let them know. You can't withdraw by declining here on Universityadmissions.se.