Lint helps track down layout layout issues, from internationalization and accessibility problems to performance issues. On every single GUI edit, lint runs incrementally on the currently edited layout. In ADT 20 Preview 3, it's even more tightly integrated into the layout editor:
Here's the screenshot again, this time with each lint-related area highlighted.
Similarly, in the layout editor itself, there are little warning and error indicators, displayed in the bottom right of each widget that has an associated problem. If you hover the mouse, a tooltip is shown with the associated editor message. Note that in this case the tooltip is shown below the layout editor window. This is done such that simply moving the mouse over the layout design does not cause tooltips to annoyingly obscure the layout you're trying to edit.
Note also that it's possible for multiple views to have lint errors "on top of each other". For example, if you look at the outline above on the right, you can see that both the button and the layout it sits inside provide errors. If you hover the mouse over an area with multiple lint warnings, they're all displayed: