A stage is a named position in your claim process — “working on signing the contract”, “waiting for the adjuster”, “ready to invoice the carrier”. It is stored on the claim with the date it entered, the date it left, who put it there and who closed it. Stages are set per company, so the list is yours rather than ours.
You edit that list under Workflow → Project stages, drag the rows into the order your office works in, and search it when it gets long. Each row shows, at a glance, what the stage does on entry, on exit, and while a claim sits in it.