A drillthrough in an SSRS report is a link to another SSRS report.

<ActionInfo>
   <Actions>
      <Action>
         <Drillthrough>
            <ReportName>FooSummary</ReportName>
            <Parameters>
               <Parameter Name="FooId">
                  <Value>=Fields!fooID.Value</Value>
               </Parameter>
            </Parameters>
         </Drillthrough>
      </Action>
   </Actions>
</ActionInfo>