The difference between indoor and indoors (or outdoor and outdoors) is very simple.

Without getting into grammatical analysis, here are a few examples to help you get a better “feel” for the words:

How to use indoor/outdoor:
  • I catch a cold easily. That’s why I prefer indoor swimming pools.
  • You can’t grow this plant in your flat. It’s an outdoor plant.
How to use indoors/outdoors:
  • As it was raining and quite cold, we decided to spend the evening indoors.
  • Playing outdoors is good for children’s physical and mental development.