Limbo: Border Jail is a technique, which is accessible through the power of the Rinnegan.
Using his Rinnegan, Madara is able to project shadows of himself into the invisible world known as Limbo (輪墓, Rinbo, Literally meaning: Wheel Grave), which is generally impossible to detect or visually perceive. The shadows can operate independently of the user, attacking and restraining target(s) or defending the original body from harm. With both his Rinnegan, Madara was shown producing up to four shadows. Madara's shadow was strong enough to repel all nine tailed beasts in a single instance, he is also capable of switching places with his shadows at any time. To the naked eye, it appeared to be the act of an invisible force. Madara prioritises keeping the technique's mechanics a secret, to enable him to use it as his trump card. However, Naruto Uzumaki gained the ability to sense the shadows' presence using his Six Paths Sage Mode, while Sasuke Uchiha was able to see the shadows perfectly with his Rinnegan.
With only one Rinnegan, Madara could only produce one shadow. Although they are invulnerable to most physical attacks, techniques which make use of Six Paths chakra can cause actual harm to the shadows. The downside to this technique is that the user's shadows, after a set period of time, automatically return to their body and are left temporarily unusable, although the recharging period seems to be a very short one. The shadows can be partially combated by immobilising or sealing the body in a way that it cannot move, act, or be recalled to the user's body. Although this does not truly defeat the shadow, it is effective at neutralising the immediate threat it poses in battle.
Both the first word's furigana Rinbo (リンボ, Rinbo) and the second word Hengoku (辺獄, Hengoku) refer to Limbo, with リンボ being the Japanese way of writing Limbo and 辺獄 being an actual translation of the word's meaning.