Entity ID is the primary key for the 'entities' table and one quarter of the primary key of the 'entity_values' table along with measure_id, date_start and date_end.
This key is used to link values in the 'entity_values' table to the particular physician, practice, nursing home, hospital or geographical location that is being measured.
It is also used to link the values to their footnotes via the 'footnote_values' table.