Tsukuyomi means "Shinto, God of the Moon".
In Naruto, Uchiha Itachi uses this as a jutsu.
It gives him the power to cause terrible tourment to a person inside their head... The jutsu doesn't follow any sort of timeline and what seems like mere seconds, could easily be many years of heavy torture... Making it an incredibly powerful jutsu. This jutsu also uses amazingly high levels of chakra and is uses very rarely, due to this.
Itachi: I just used Tsukuyomi on him... He has spent the last 5 years reliving his family's death.
Kakashi: You're sick.
Tsukuyomi: literally "Moon Reader", named after the Shinto moon god, Tsukuyomi. One of Uchiha Itachi's kekkei genkai techniques enabled by the Mangekyou Sharingan, Tsukuyomi is perhaps the most powerful visual genjutsu ever used. Simply by making eye contact with the victim, Itachi can use Tsukuyomi to transport them to an illusionary world of Itachi's creation. This world seems to have several constants: a dark crimson sky with a full moon and black outlines of objects and/or people seen inside it. Because Itachi created this alternate reality, his control of it is absolute, including the flow of time itself. Limited only by his own imagination, he is able to torture an opponent in any way he desires, leaving them crippled and in a state of severe mental collapse. This technique also requires a large amount of chakra to perform, which limits Itachi's use of it.
Kakashi: *falls to the ground after enduring three days of continuous torture in the space of a moment*