noun
player depth; pool of players
Used in sports to describe the overall quality and quantity of players available in a team, especially beyond the starting lineup.
このチームは選手層が厚いので、けが人が出ても大丈夫だ。
This team has deep player depth, so it's okay even if someone gets injured.
優勝するには、選手層の充実が欠かせない。
To win the championship, a solid pool of players is essential.
層 means 'layer' or 'stratum'; 選手層 combines it with 選手 (player) to mean the 'layer' of players available.
Compound of 選手 (player) and 層 (layer, stratum). The term is a modern sports expression with no known historical derivation beyond the combination of its components.