"Into the TARDIS" isn't actually a standalone keyword—it's flavor wording on the DoctorWho Universes Beyond cards, most notably the saga "Into the TARDIS." Mechanically, it's a Saga enchantment that exiles permanents you control and returns them later, representing the TARDIS being "bigger on the inside." When the chapter abilities trigger, you exile chosen permanents, then bring them back together on a later chapter, effectively flickering (blinking) them.
The common mistake: players think exiled cards leave permanently or that they choose what returns freely. They don't—the saga tracks them, and they return at the designated chapter, re-entering as new objects with new ETB triggers and no prior counters or auras.
In Commander, this matters for protecting permanents from removal, resetting creatures, and re-triggering enters-the-battlefield effects across your board for value.