Android manages its memory by automatically closing apps and processes that are not needed, while keeping in memory the ones that you use more often.
Closing applications on your Fairphone 2 will likely not give you a performance boost. 

If you want to manually close on of the apps, here is how to do so:

1. Touch the open apps button (the square icon on the bottom right side of your touchscreen).
2. Swipe left and right to find the app you want to close
3. Swipe up to close the app.
   You can also close all the apps by swiping to the beginning of the list and touch "Clear All".