When "interactive mode" option set on in the "Run Tests" modal window Puppetry runs the tests in the browser. It renders a panel with the suite tree on top of the pages. The tests pause on the first meaningful test step waiting until you click on the Next button in the panel. As you click the next step performs and the cursor in the panel moves respectively. This way you can navigate the test cases and check on-the-fly what actually happens with every test step. In order to make it even more obvious Puppetry highlights the targets involved in the action.
You can open DevTools in browser by pressing ⌘⌥I (Mac) or F12/Control+Shift+I (Window/Linux)