diff --git a/src/db/UniqueTags.hxx b/src/db/UniqueTags.hxx
index d2a7eb734..8f26d4bb4 100644
--- a/src/db/UniqueTags.hxx
+++ b/src/db/UniqueTags.hxx
@@ -29,6 +29,9 @@ struct DatabaseSelection;
 template<typename Key> class RecursiveMap;
 template<typename T> struct ConstBuffer;
 
+/**
+ * Walk the database and collect unique tag values.
+ */
 RecursiveMap<std::string>
 CollectUniqueTags(const Database &db, const DatabaseSelection &selection,
 		  ConstBuffer<TagType> tag_types);