It's a blowjob, but not just any typical blowjob. The main difference is that the blowjob is given by two people instead of just one. The two people acts like the two slices of bread while the reciever's cock is like the meat sandwiched between the two slices of bread. Thus, the name "cockmeat sandwich" and always involve at least 3 people.
From 2008 movie
Harold & Kumar Escape from Guantanamo Bay.
This term was used when Big Bob (a guard in Guantanamo Bay) asked Harold & Kumar to get hungry real fast and eat his cockmeat sandwich with extra mayo.