how do teams retain players mean.. ??

These players play for team is last season that players team should allowed to retain like - Cheenai Super Kings (CSK) retain 1. M.S.Dhoni 2. Suresh Raina 3. Ravindra Jadeja Royal Challengers Banglore (RCB) retain 1. Virat Kohli 2. Ab De Villiers (Mr.360) 3. Sarfaraz Khan Mumbai Indians (MI) 1. Rohit Sharma 2. Jasparit Bumrah 3. Hardik Pandeya