Player Tiers are a way of advancing a player's OVR beyond 180%, which provides a clearer and more meaningful player progression in higher stages of player development. Player Tiers give a permanent increase on key attributes of a player. Increasing a player's OVR above 180% or a single Attribute above 400% can be achieved only through Player Tiers. As soon as a player reaches the 10th star, the first ‘Rare’ Tier unlocks. 

There are 5 Tiers, each gradually increasing the player's key attributes once it's reached.

Once you tier up your player, the Key Attribute increase will stay until his retirement, no matter the OVR decrease of 20% after a season maintenance. 

To advance a player to a higher Tier you'll need Gems. Check out the link to learn more about what are Gems and how to use them.