This is pretty easy to understand. You have a grid which is labelled vertically and horizontally with letters and numbers. When finding positions, you could be asked to write down the position where an object is placed, example: Red planet = A7. Else, you could be asked to say what object is to be found in a certain given position, for example: A7 = red planet.

When working with positions, we are given a grid that looks like the one here. So, for example: 

* What position does the red circle have? 
   Answer = B3 

* What can you find in B5?
   Answer = The sun