The Kinsey scale, which is referred to by sex researchers as the Kinsey continuum, is a way of thinking about sexual orientation. Prior to Kinsey the widely accepted scientific understanding of sexual orientation was that a person was either heterosexual (if they were sexually attracted to people of a different gender) or homosexual (if they were sexually attracted to people of the same gender.
Kinsey and his colleagues proposed that sexual orientation was in fact on a continuum, with people who are exclusively heterosexual on one end and people who are exclusively homosexual on the other. However they believed that few people existed on either end of the continuum, and that most of us were somewhere in the middle, more or less to one side. Bisexuality was thought to be the midpoint of the scale, which had seven points on it.
The Kinsey scale is still referenced in sex research today, although increasingly flexible and complicated models of sexual orientation have been developed since Kinsey introduced the idea of the continuum in 1948.