All cotton is carded, a step that opens up the fibers and roughly lines them up. Combing is an extra pass with fine combs that removes short fibers, neps (tiny tangles) and leftover plant matter. The longer fibers that remain make a cleaner, more uniform yarn with fewer loose ends poking out.
In a tee, that means a softer, smoother surface, less fuzz and usually less pilling, which also gives prints a better base, especially fine detail and DTG. Combing adds cost because it takes time and discards some fiber, so it shows up mostly in retail-style blanks.
Spec sheets often say combed and ring-spun together, but they are separate steps: combing cleans the fibers, ring spinning twists them into yarn. A blank can have one without the other, and carded open-end cotton is the economical baseline. More in the cotton yarns guide.