Tiers are groupings of players intended to help with draft strategy. For example, in 2023 Patrick Mahomes, Josh Allen and Jalen Hurts are commonly being ranked in the top tier of QBs, while Joe Burrow, Lamar Jackson and Justin Fields may make up your second tier, and so on.
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.