Isfet means "injustice" and "wickedness". It's the opposite of Ma'at, which is order. Isfet is an abstract concept. Both concepts can't exist without each other. At the bottom of the symbol, a figure which vaguely looks like a giant serpent can be seen, which could refer to Apophis, the manifestation and embodiment of chaos itself.
Isfet is mentioned at the start of Serpent's Shadow when Apophis leaves the burning hieroglyph on a grassy field following his attack on JD Grissom's King Tut exhibit. Isfet is the force that pulls Carter and Zia Rashid toward the Sea of Chaos. Carter describes Isfet as a blazing hieroglyph which no magician would ever write. Later when the magicians are fighting Apophis, Carter sees Isfet inside the snake's hood.