diff -u b/modules/field/modules/text/text.module b/modules/field/modules/text/text.module
--- b/modules/field/modules/text/text.module
+++ b/modules/field/modules/text/text.module
@@ -229,7 +229,7 @@
       '#size' => 10,
       '#default_value' => $settings['trim_length'],
       '#element_validate' => array('element_validate_integer_positive'),
-      '#description' => t('Limit the display of this field to a certain amount of characters.'),
+      '#description' => t('If the summary is not set, the trimmed %label field will end at the last full sentence before this character limit.', array('%label' => $instance['label'])),
       '#required' => TRUE,
     );
   }