A Work Item is Serpent's unit of change: one feature or fix, with its Git branch and tracked scope set up for you. "Work Item" is a Serpent term, not standard Salesforce vocabulary, so it is worth learning first.
Think of it as a ticket and its branch combined: you work on the Salesforce changes while Serpent handles the branch, review request, and release path. See How Serpent Works: Key Concepts for the full picture, or Create a New Work Item to try it.