What does the icon to the left of the object name in the outliner mean? I have been having a problem with "can not operate boolean operation" to unite 2 objects. One is definitely mesh and other one, I thought it was mesh but it has a different icon next to it. I did roll over it but it did not tell me what it meant. I suspect this needs to be converted to a proper mesh to boolean. 

It is the icon in the outliner panel immediately to the left of the stem final copy and stem_final objects. It is a file imported from sketch up, with the file format "dae". I also converted this file in another software "mesh lab" obj file, ply file, neither worked. One file imported from mesh lab in the form of dae worked as mesh, so I am not sure what is going on...



When I try to find out if the object is non-manifold with Ctrl+Shift+Alt+M, all the lines light up in orange…
If you examine the "add objects" menu, visible by using the SHIFT-Akey combination, or the  Spacebar > Akey sequence, you will see that the icon to the left of "stem final copy", and "stem_final" in the excerpt of the image of the outliner panel you link to is used for one of three object types: an empty object, a force field, and a group instance. None of the three is a mesh, and so cannot be part of a boolean operation, as these take two mesh data. 

You can create something equivalent to an empty by adding a mesh object, and in edit mode selecting and then deleting all of its vertices, but I don't see in the Blender Manual any information on converting an empty, force field, or group instance to a mesh.