What is a membrane sweep?

My gynae did it for me when I was about 39weeks. It is quite similar to vaginal examination, to check on how many cm has been dilated. It was quite painful and uncomfortable for me. My gynae told me it helps to stimulate and kick start the labor (dropping of mucus plug). After the membrane sweep, I did get some brown discharge. However, my real labor only came after 4-5days post membrane sweep.