A cabana is a small, sometimes portable changing room near a swimming pool or beach. You may put your bikini on in a cabana before diving into the cold ocean water.
A traditional cabana is a tent that can be moved from one spot to another on the beach. It's a mobile dressing room, where swimmers can dry off and put their clothes on. You can use the word cabana to describe any poolside changing room or building — it comes from the Spanish cabaña, with its Late Latin root capana, "hut" or "cabin."