I did a Google Search and searched thiese boards but I could not find anything that will explain what this "Common Craft" skill is that my Human Fighter/Rogue has.
Create Common Item, a common crafting skill, has been added. It is now possible for characters other than Dwarves to produce items using the Create Common Item skill. Commonly produced items are new items distinct from those that are normally produced by Dwarves.
Hair accessories, potions, and dyes can be produced with ordinary recipes and items caught by fishing. Adding a recipe to the Common Recipe Book is identical to that of Dwarves.
The Create Common Item skill is automatically acquired as character level rises..."
This is taken from the News/Latest Chronicle/Chronicle 4/Skills tab on this site. I don't have any personal experience with it, but from what I understand you can acquire materials and recipes through fishing that you can use to create items such as healing potions and other things that can easily be obtained in other ways.