plone.app.search combines search results listing with Advanced Search form. This provides search results view for Plone CMS with possibilities to filter, sort and apply advanced filters to the search results without. When changing a search criteria on the search view, AJAX call is delivering you the updated results instead of reloaded page that lets you get to what you're searching for faster without much distraction caused by page reloading. The package is part of Plone 4.2.
plone.app.search also updates main search field (usually located at the top right of your Plone site by default) and search portlet to return results in the updated listing view.
plone.app.search has the aim to give intuitively clear and understandable interface for searching in Plone. But there might be some new things that are not so obvious. Let's see how the search results works now.
This package is merged into Products.CMFPlone in Plone 5.