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 that are lower in the hierarchy. God Cards and the same level of the hierarchy are vulnerable to each other's effects.
This also extends to the use of the effect of "Obelisk the Tormentor" to increase its ATK to ∞.
"Slifer the Sky Dragon" and "Obelisk the Tormentor" have a position of 1 in the hierarchy, while "The Winged Dragon of Ra" has a position of 2. In the Yu-Gi-Oh! R manga, "The Wicked Dreadroot" and "The Wicked Eraser" have a position of 1, while "The Wicked Avatar" has a position of 2.
The card "Divine Evolution" can increase a God Card's position in the hierarchy. Yami Yugi used this card on "The God of the Obelisk" (this enhanced form known as "Obelisk The Progenitor"), making it able to destroy Yako Tenma's "The Wicked Avatar" by battle (both monsters had equal ATK, so both were destroyed).