Your Tier in a Progression Path represents your level of experience and progress on it, from 1 to 5.
You have a separate Tier number for each Path. The first Perk you purchase puts you at Tier 1. Every 2 Perks in the same path you have increases your Tier there by 1, increasing your effectiveness and unlocking Perks at higher Tiers.
Higher Tiers can also increase the damage, range, etc. of your Abilities in that path.
