This module aims at making facets filterable on field values stored on the facet items.
It is comparable to the "Exclude specified items" filter which is part of the facetapi_bonus module.
Exclude specified items filter lets the user exclude items based on there #indexed_value or #markup.
Facetapi field filter lets the user exclude items based on field values stored on them.
In case the field is actually a fieldable entity, any fields on that entity will be added to the list as well.
Let there be two entity types, "Activity" and "Chore".
An "Activity" consists of a number of "Chores".
Let there be a field called "Priority" on the "Activity".
Possible priorities would be: "High", "Medium" and "Low".
In the facet you only want to list the "Activities" with priority "High".
Selecting an "Activity" would show the list of "Chores" associated with it as usual, but the activities are limited to the ones that are high priority.