diff --git a/reference/rrd/functions/rrd-graph.xml b/reference/rrd/functions/rrd-graph.xml
index 5e4838a18e..809b192965 100644
--- a/reference/rrd/functions/rrd-graph.xml
+++ b/reference/rrd/functions/rrd-graph.xml
@@ -26,7 +26,9 @@
     <term><parameter>filename</parameter></term>
     <listitem>
      <para>
-      RRD database file name.
+      The filename to output the graph to. This will generally end in either
+      <literal>.png</literal>, <literal>.svg</literal> or
+      <literal>.eps</literal>, depending on the format you want to output.
      </para>
     </listitem>
    </varlistentry>