Refers to the level of the pilot skill tree you have attained, or the level that you are recommended to have before fighting an enemy. NPC ships in space also have tier numbers under their name to determine their approximate difficulty level.
This also effects your GCW points earned for killing a NPC of the opposite faction in space (you can be a freelance pilot and your faction on the ground determines if you get points for the kill, being freelance just makes them not aggro you)
Tier ranges from 0-5. When you start a Pilot profession, you are at Tier 0.
-When you have just finished your 1st trainer you have Achieved Tier 1.
-When you have just finished your 2nd trainer you have Achieved Tier 2.
-When you have just finished your 3rd trainer you have Achieved Tier 3.
-When you have just finished your 4th trainer you have Achieved Tier 4.
Once you have mastered the pilot profession, you'll be considered Tier 5.