A Cum Trough is a word (albeit a crude one) that describes the indentation found in the mid back area of a woman. Its usually a line that runs down the length of the spine (disappears at the lower back) and is sunken in more than either side of the woman's back. Usually it isn't found in really thin women (usually you just see their actual spine) nor heavyset women (whose backs are usually just doughey and flat). Most men, consciously or not, find this part of the body sexy.
The reason this area is called a Cum Trough is because when engaging in doggy style sex or a facsimile thereof, the man may be so inclined to pull out of the woman and ejaculate on her mid back. Rather than just leaking every which way, the Cum Trough acts as a trench or trough of sorts and causes all the semen to stay in a neat straight line on the woman's back, presumably until she stands up/rolls to either side.
Joe: Yeah dude, my girl is great. Oh and to top it all off, she has a nice little Cum Trough that I got to try out last night.
Bob: Yeah you're lucky, I keep going out with these damn skinny women, they have nothing but spine. Where is a man to deposit his seed, I ask you!
Bob: Oh yeah... I'll get back to you on that.