The position is the order that the tracking code will prioritize and swap for the tracking source and tracking number on your website. The lower the number, the higher the priority.
For example, CTM offers presets for both Referrals and Facebook tracking sources. Technically any website that points to your website is a referral. CTM can separate Facebook traffic from all referrals with the use of a lower position. In the tracking source preset for Facebook the position is set to 4 while the referral tracking source position is set to 14. This allows CTM to prioritize swapping for Facebook traffic.