diff --git a/doc/src/sgml/pgstattuple.sgml b/doc/src/sgml/pgstattuple.sgml
index 4071da4ed94..c747a5818ab 100644
--- a/doc/src/sgml/pgstattuple.sgml
+++ b/doc/src/sgml/pgstattuple.sgml
@@ -270,6 +270,15 @@ leaf_fragmentation | 0
      page than is accounted for by <literal>internal_pages + leaf_pages +
      empty_pages + deleted_pages</literal>, because it also includes the
      index's metapage.
+     <literal>avg_leaf_density</literal> is the fraction of the index size that
+     is taken up by user data.  Since indexes have a default fillfactor of 90,
+     this should be around 90 for newly built indexes of non-negligible size,
+     but usually deteriorates over time.
+     <literal>leaf_fragmentation</literal> represents a measure of disorder.
+     A higher <literal>leaf_fragmentation</literal> indicates that the
+     physical order of the index leaf pages increasingly deviates from their
+     logical order. This can have a significant impact if a large part
+     of the index is read from disk.
     </para>
 
     <para>