Cannon Fodder is any unit or group of units whose most important function is to draw fire from other units.
Cannon fodder units are commonly cheap (though preferably solid) units. They are usually located at the front or around an attacking force and will draw fire from attacking units which might otherwise target fragile, but valuable units like artillery or missile launchers. Due to their fragility T1 Attack Bots are poor cannon fodder against units with rapid fire capabilities. A better substitute is probably T1 tanks.
Cannon fodder can be used with intent or stuck upon incidentally. Pillars can be used as Cannon Fodder for Flapjacks to protect them. Mech Marines are only useful as Cannon Fodder against Auroras because they are so terrible against them.