This directory is responsible for storing the store feed while it is being imported. The plugin will try to guess where your temporary directory is and fill in the proper path. If for some reason your server's path to the temporary directory cannot be automatically deduced, you will have to create a directory in your "home" folder and make it writable (CHMOD 755 or 777, depending on how php is configured on your server). If this directory is located within the web root ("public_html", "www" or similar), you should also add a "Deny from all" .htaccess file to it.