Lockdown, known as Solitary Confinement in Japanese versions, is a mechanic in Persona 5.
Lockdown is unlocked when Strength Confidant reaches rank 3. It allows a Persona to stay in the Velvet Room for training new passive skills for covering its own weaknesses. The exact days required for training depends on the player's Confidant rank of correlated Arcana. The higher Confidant rank is, the shorter time is required.