The phrase "visible ribs" means that the bones in the chest area, called ribs, can be easily seen because there is very little fat or muscle covering them. Full definition
Although my legs appear 20 % better now from when I began that routine, my upper torso looks much too thin (visible rib cage). (rachaelattard.com)
Some of the racing breeds may have visible ribs as they mature but this will normally be just the last two or three ribs, no more. (thehappypuppysite.com)
Tip: Some breeds have more visible ribs than others, and it's harder to feel the ribs of dogs with long or thick hair. (trupanion.com)