hstore is a postgresql module, which is used optionally by newer versions of xowiki and xowf (xowiki content flow) to provide quick access to the "instance_attributes" (the per-form or per-workflow attributes not part of the native content repository data model). The supporting functions (creating indices, etc) are currently mostly part of the xowiki sources (earlier: xowf).
Under Ubuntu, you can install hstore via:
psql -d <yourdb> -tAc "create extension hstore"
You can then verify the successful installation on the ds/shell using: