Comes from the basic Computer Science principals of High Cohesion and Low Coupling.

High cohesion means the functionality of a component are closely related and focused on performing a single well defined task. Basically single classes for single purposes.

Erosion of this is when classes start doing to many things, in the case of God classes.

The Change Impact formula looks at a way of detecting when the cohesion is eroding and flagging it on a change (as the pull/merge request itself should generally be single focus cohesive change)