Kassa (a.k.a space corn, or evil Orville Redenbacher, according to Lt. Colonel Cameron Mitchell) is a crop that was genetically engineered by the Lucian Alliance. Kassa looks very similar to maize (corn) on Earth, but has a pink/purplish color rather than yellow, like it's normally grown on Earth. The crop has been engineered with a psycho-stimulant, making the crop highly addictive to whomever eats it.
The crop first came to SG-1's attention in 2006 when SG-3 explored a planet where the inhabitants behaved strangely and rioted when the supply of Kassa ran low. Lt. Mooney had a taste and quickly grew addicted to it. Colonel Albert Reynolds had to restrain him and bring him back to Stargate Command. Two thirds of the known Kassa crop is grown on P6G-452. SG-1 then made it a duty to rid the galaxy of the crop once and for all. (SG1: "Off the Grid")
Lucian Alliance freighters are ships used by the Lucian Alliance as transport vessels to carry Kassa into Lucian Alliance territory. These trains are made up of at least four massive cargo containers with Transportation rings, which the Kassa is stored in. In 2007, SG-1 destroyed three of these trains by ringing aboard, via cloaked Tel'tak, and planting an explosive charge in each container. The Al'kesh escorting the containers was destroyed as well. (SG1: "Bounty")