Lockdown, known as Solitary Confinement in Japanese versions, is a mechanic in Persona 5.
Lockdown is unlocked when the 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 - at max rank, it takes 2 in-game days for a Persona to finish its training.
While a Persona is in lockdown, it cannot be used in battle, gain experience, or be used for fusion until the player retrieves it. If it remains in lockdown for too long, the Persona is lost - however, Caroline and Justine will inform the player when they reach the deadline for releasing their Persona. Accessing the Velvet Room doesn't consume a time slot, so the player has nothing to lose in collecting their Persona, but be careful if plot-important events forcibly cause the player to advance time past their deadline.
The skill learned from lockdown will, most of the time, be specified to cover a weakness - for instance, a Decarabia is more likely Dodge Phys or Resist Phys than other resistance passives. If a Persona has multiple weaknesses, the skill it learns will select a random weakness to cover. The type of skill to be gained is decided once the Persona commences its detention - if the Persona picks up the wrong skill, reloading to just before release will not change it.
At Strength Confidant rank 8, Personas left in Lockdown will pick up better resistance passives. This includes Null, Drain, Repel, and Evade.