This is the sensitivity to light of a particular type of film. It is the specific number used to measure film speed. Your film will list an E.I. number on the box or the film can as the film speed. It is the same as A.S.A. and I.S.O. on your light meter.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission