The exporter is a python script that converts Blender meshes to a special file format called .nmf so that the mesh can be loaded into ATI's normal mapper utility. The normal mapper takes a high polygon count mesh and a low polygon mesh and genenerates a normal map either in tangent space or object space. You can then use the normal map along with your favourite normal map shader and apply it to the low poly mesh in Ogre to make it look as detailed as the high poly mesh.
The script includes a GUI so that you can configure the settings for the normal mapper within blender and then build the normal map while you continue to work within Blender. Included in the zip download is a modified version of the normal mapper (including source code) that supports more image formats by incorporating the use of the Devil Image library. Now supported are png, jpg, bmp, and of course tga and all others supported by Devil. There is a bug with using bump maps in ATI's normal mapper which generated inverted normal vectors for the bump map. This has been fixed in the modified version. So its strongly recommended to use the modified version included in the download.