Why Check and Resolve Warnings?
Revit checks things as they are created or changed, often a warning will pop-up at the time.
It keeps a record of all these warnings and the identity of the objects effected.
Some warnings are benign, and are merely helpful (like line slightly off straight), other warn you that schedule data may be incorrect (object not in room), but some are reporting your model is not consistant (one object is completely inside another).
It is the last type that you must resolve. These have the potential to corrupt your project file.
The other important reason is to keep the size of the Warning list as small as possible. Whenever you open or save warnings are checked to see if they are still relevant. A long list (many hundreds) can take a noticable time.
Where are Warnings?
Warnings are under the Manage tab, Enquiry ribbon.
If it is greyed out there are no warning to report.
To see how many warnings there are go to the last item in the list, click on the + symbol and note the number of the warning. That is how many there are.
How to Identify Objects with Warnings
The Warning dialog has a Show button for objects you have ticked.
If you note down the ID numbers involved you can also use Select by ID, also under the Manage tab, Enquiry ribbon.