To sit down — to lower one's hips from a standing position into a seated position. Similar to '座る' and commonly used in spoken Japanese.
バスが来るまでベンチに腰を下ろして待った。
I sat down on the bench and waited until the bus came.
疲れてその場に腰を下ろした。
I was tired and sat down right there.
客が来て座布団に腰を下ろすと、お茶が出された。
When the guest sat down on the zabuton, tea was served.