In addition to writing tags, ratings, labels, descriptions, etc. directly into the photos, version 2.0 of digiKam can save metadata in a separate .xmp file. This approach has several advantages. First off, it speeds things up, as writing data to a text file is faster than embedding metadata into photos. This also allows you to store metadata for RAW files, since writing metadata directly to RAW files can sometimes be problematic.
The feature that allows digiKam to store metadata in .xmp files is called XMP sidecar. To enable it, choose Settings » Configure digiKam, switch to the Metadata section, and select the desired option from the Metadata Writing Mode drop-down list. You can choose between Write the XMP sidecar only and Writer to image and XMP sidecar. Tick then the Read Metadata from XMP sidecar files check box, hit OK to save the settings, and you are done.