ImageSpace SMQTK is a plugin for utilizing similarity searches and Interactive Query Refinement.
Installation of this plugin is the same as any other, documented in the installation README.
IMAGE_SPACE_SMQTK_NNSS_URL points to the SMQTK nearest neighbors service as a URL in the form http://somehost.com.
IMAGE_SPACE_SMQTK_IQR_URL points to the IQR service provided by SMQTK, this is a url in the form http://somehost.com.