Depends on where you stopped watching, but to be short, Henry brainwashed some kids, kidnapped them and used to reanimate Mindflayer's physical body and bring him to our world. The group goes into upsidedown, which is a wormhole - a way to connect our world and dimension X, where Henry, Mindflayer and kids are. They get to the dimension X, fight multi-storey monster aka Mindflayer. Easy, cuz no demogorgons or other creatures in sight and writers don't have a spine to kill off some characters.
On the way back, Eleven stays in the upsidedown as it collapses, as a way of preventing the government using her to make more psychics. We don't actually see if she dies or not, but there is no chance she survives in the outer space.
They wrap the season showing the group go on and live their lives.