diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 808e1e3..3dcde61 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1045,7 +1045,12 @@ protected function curlExec($curl_options, $redirect = FALSE) {
       $curl_options[CURLOPT_HTTPHEADER][] = 'Expect:';
     }
 
-    // Forward xdebug session.
+    // In order to debug webtests you need to either set a cookie or have the
+    // xdebug session in the URL. If the developer listens to connection on the
+    // parent site, by default the cookie is not forwarded to the client side,
+    // so you can't debug actual running code. In order to make debuggers this
+    // bit of information is forwarded. Make sure that the debugger listens to
+    // at least 3 external connections.
     if (isset($_COOKIE['XDEBUG_SESSION'])) {
       $curl_options += array(
         CURLOPT_COOKIE => '',