Tiers are groupings of players intended to help with NHL fantasy draft strategy. For example, you might rank five centers in Tier 1, seven centers in Tier 2, four in Tier 3, etc in your fantasy hockey rankings. Generally speaking, the players in a particular tier can be considered interchangeable, with any one of them considered to be a reasonable selection ahead of any of the others in the same tier.
Keeping track of these groupings allows you to monitor for potential drop-offs in the quality of players at a specific position.