The ExpandProperties simply replaces property names with their property values. For example, if you have the following in your build file:

<property name="description.txt" value="This is a text file" />

<copy todir="/tmp">
  <filterchain>
    <expandproperties />
  </filterchain>

  <fileset dir=".">
    <include name="**" />
  </fileset>
</copy>

And the string ${description.txt} it will be replaced by This is a text file.