Hierarchy is a term used to describe the rank of the Egyptian God Cards and the Wicked Gods. God Cards are not affected by the effects of God Cards with a lower hierarchy. This also extends to the use of the effect of "Obelisk the Tormentor" to increase its ATK to infinite. God Cards of the same hierarchy are vulnerable to each other's effects.
"Slifer the Sky Dragon" and "Obelisk the Tormentor" have a hierarchy of 1, while "The Winged Dragon of Ra" has a hierarchy of 2. In the Yu-Gi-Oh! R manga, "The Wicked Dreadroot" and "The Wicked Eraser" have a hierarchy of 1, while "The Wicked Avatar" has a hierarchy of 2.
The card "Divine Evolution" can increase a card's hierarchy, as shown when Yami Yugi's "Obelisk The Progenitor" ("The God of the Obelisk" affected by "Divine Evolution") was able to destroy Yako Tenma's "The Wicked Avatar" by battle (both monsters had equal ATK, so both were destroyed).