Each Star on a particular piece of equipment represents the number of slots initally on said piece of equipment. You can add slots (which do not change the equipment's star count) onto a piece of equipment by purchasing and using a Super Sea Snail (from the Alley for 30,000 coins), or earning them after a Splatfest. Gear bought from Spyke will arrive with a number of gear slots matching the number of stars it has. This is significant as other players tend to add slots to their gear.