This is a sealing technique based on the fūinjutsu of Uzushiogakure. The sealing formula is used to seal any target into a human body or an object, and is mainly used when a giant enemy or evil spirit needs to be sealed away. To use it, it is necessary for the user to have great ability.
When two Four Symbols Seals are used together, they form the Eight Trigrams Sealing Style. When Minato sealed the Nine-Tailed Demon Fox into Naruto Uzumaki, he left an opening at the space where the two four-symbol seals meet, allowing the Nine-Tails' chakra to merge with Naruto's chakra. Minato also left behind a key with Gerotora, which would be used to tighten the seal when it weakened. Naruto would eventually use the key to gain control over the Nine-Tails' chakra.