The term Random Greens in World of Warcraft refers to items of Uncommon item quality which are listed on the loot table of any given mob. These items possess a given drop rate, measured in percent, and at the time the mob is killed, chance dictates whether or not the item drops. Also known as world or random drops, these items usually have the capability of dropping off of a large number of different mobs, and usually have a fairly low drop rate overall.
These items differ from static drops. Static items come typically come from instance bosses, or at the very least have a distinct name and a particular assortment of attributes. Random greens will be created with one of various suffixes, which determine which actual attributes are found on the item. Static items are also much more likely to be of a higher item quality, whereas blue or better random items are incredibly uncommon and usually only come as an additional portion of a boss drop. (Example: Ribsplitter has 10 attack power, plus some random suffix bonus.)