If you look at your Dock some day and see a big question mark where an application icon is supposed to be you are probably going to wonder what it means and what to do about it.
The icons resting on your Dock are alias’ that link to the location on your hard drive where the applications are actually stored. In most cases that means your Applications Folder.
If you recall, you added something to the Dock by dragging the icon image from the location folder (Applications) to the dock and released it. That “links” the alias icon to the application itself.
When that big blue question mark suddenly shows up, that means the alias icon on your Dock can no longer find the application. It has lost it’s reason to exist and says “help, why am I here?”
You might as well pull it off the dock and release it because it no longer functions.
What to do about it depends on what caused the problem in the first place. If you discarded the application, then throwing away the question mark solves the problem. Everything is neat and tidy and you are ready to move on.
If you moved the application somewhere else, such as to a backup external drive or just to some other folder, then find that location and pull an alias icon back down to the dock and you should be set to go.