@swewe it can refer to a center/centre (like an organization that provides a quiet and remote place for people to relax and enjoy a calm atmosphere).
However, it could also refer to any place that the person goes to for the purposes of a retreat. For example, someone might camp alone in the forest/woods as their retreat.
So doing a retreat basically just means going somewhere that is calming and relaxing and that doesn't have a lot of noise or people around.
Note: It does not simply refer to a place where you are able to rest/sleep (like a hotel). It refers specifically to a place where people go for calmness, peacefulness, relaxation, etc. (often to improve one's mental health).