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.