You should be able to see SOME light between the barrel and cylinder on ALL revolvers (this is called the Barrel/Cylinder gap, or B/C gap). If the front face of the cylinder is actually touching the rear of the barrel (no light through gap at all), then the two surfaces will rub when the cylinder rotates, which can get worse when the weapon is hot and/or dirty. This rubbing/drag will make it difficult to thumb-cock the revolver, and even more difficult to shoot it in the Double-Action (DA) mode (just pulling the trigger), as the trigger has less leverage to rotate the cylinder than the hammer does when thumb-cocking.
When the weapon is clean and cold, most revolver B/C gaps will probably fall between .004 inches and .012 inches. Smaller than .004", and the cylinder may drag when hot or dirty; much larger than .012" and you'll be losing more gas pressure than normal through the gap when firing, and getting more flash and "spitting" of hot gunpowder particles, too. Sometimes accuracy will be poor on revolvers with larger B/C gaps (usually with softer lead bullets), but not always. You can measure the B/C gap with a set of precision feeler gauges.