"Ebony" refers to the ethnicity or skin color, in this case, black. So an "ebony" themed video typically involves black performers.
"Backshots" is a term used for a particular type of video. In a backshot video, the focus is on the buttocks of the performer(s), as opposed to their faces or frontal nudity. Backshots are popular because they often provide a unique and provocative perspective that many viewers find attractive.
So, "ebony-backshots" would be a porn video featuring black performers where the focus is primarily on their buttocks.