"Wide lats" refers to well-developed and muscular latissimus dorsi muscles, which are located on the sides of the back. It describes having a broad and strong upper body appearance. Full definition
In this case, a wide grip won't necessarily build wide lats — in fact, studies have shown that the combination of a pronated wrist position and a close neutral grip will allow for maximal activation of the lats on pull - downs. (fitnessandpower.com)