Gordo is Spanish for “fat.” In cigar shops it refers to the thickest cigars, usually with a 60 ring gauge or more, often about 6 inches long. Some companies use the word for cigars with a ring gauge in the mid-50s and up, and others sell cigars thicker than 60 under their own names. It is mostly a modern New World size rather than a traditional factory size.
A wide ring gauge means there is much more filler compared with wrapper in each puff, so a gordo can smoke cooler and taste different from a thin cigar made with the same blend. It also takes longer to smoke and can be awkward to hold and to cut; make sure your cutter opens wide enough for the cigar. None of this makes it better or worse; the differences come from the size and shape.
Compare the toro (usually about 50–54 ring gauge) and the double corona (long rather than wide). Sizes: ring gauge and length cheat-sheet.