Every deal has a Checklist tab in the right-hand sidebar, next to Tasks. It lists items tied to that deal — each one shows whether it's checked, the date it was checked, and, when known, whether it was checked automatically by a workflow running on the deal or checked by a person.
If the deal has no checklist items yet — for example, no automator has checked anything off on it — the tab still shows up, just with a short note explaining that items will appear here once an automator checks something off on this deal.
This tab is currently read-only: you can't check or uncheck an item yourself, or add new ones. Most checklist items get checked automatically as a workflow reaches that point — an Action step that checks an item off, or a Task step linked to an item, which checks it off the moment the task is marked done (for an Approval, the moment it's approved) — or, if the run was paused, on hold or needed attention at the time, once it picks back up. See Running an automator on a deal for how those workflows progress. It's different from the Tasks tab, which lists work assigned to a person — see Tasks on a deal.