It means you play offline with a friend on the same console and the same screen. It can be either multiplayer or campaign, usually.
The word co op is short for cooperative. This word can be used in many different settings. It generally is used to describe a working partnership between two parties.
co-operation mode
(XBOX) You Can Only Do Co-Op on Special Co-op Missions. To do Co-op Go to the Multiplayer Menu and hit Co-op
What are of features of co-op banks?
There's no co-op in the Single Player Campaign but there is co-op in Specials Ops.
Yes, there is co-op in Halo Reach.
Co-op Brewery was created in 2006.
The population of Saskatoon Co-op is 900.
Code Co-op was created in 1996.
Co-op Atlantic was created in 1927.
Sven Co-op happened in 1999.
The population of Phone Co-op is 40.
Phone Co-op was created in 1998.
Breaks Co-Op was created in 1997.