I found a reference hidden on the same slide: Fraser, A. G. & Marcotte, E. M. Nat Genet, 2004, 36, 559-564 .
Im not an expert in biochemistry, but I guess its the interaction between proteins. Since my lecturer mentioned that the proteins that worked together where clustered together.
I was really only interested in the software, since I found another use of it. If it works I will "publish" it on my blog, than you see...