The position is a number you assign to be able to more logically order your fixtures in the list. Sometimes fixtures aren't patched in a logical order and to ease working with them in some more logical order, you manually assign them a position number. It's really there just to help you.
A more concrete example is when you have 10 groups of 50 fixtures each and you want to be able to quickly select the 1st, 2nd, 2rd, etc fixture of each group. You can then assign them ascending position numbers, restarting at 1 for each group.