They all refer to the thickness of the blade, from the spine to the cutting edge. For instance, and 8/8 blade, would be a full inch from the spine to the cutting edge.

Most beginners prefer 5/8 or 6/8 blades. They can be easier to maneouver around your bones and skin. As you become more experienced, you may find one or many sizes that work for you. I prefer 6/8 and over, specifically 7/8 and 8/8 blades. I like the flex from the larger blade, and the weight from the whole thing.

Try a bunch of different sizes, and you may well feel a difference.

Good luck.