--- a/testing/marionette/harness/marionette/runner/base.py
+++ b/testing/marionette/harness/marionette/runner/base.py
@@ -247,17 +247,21 @@ class BaseMarionetteArguments(ArgumentPa
             if not os.access(path, os.F_OK):
                 os.makedirs(path)
             return path
 
         self.argument_containers = []
         self.add_argument('tests',
                           nargs='*',
                           default=[],
-                          help='Tests to run.')
+                          help='Tests to run. '
+                               'One or more paths to test files (Python or JS), '
+                               'manifest files (.ini) or directories. '
+                               'When a directory is specified, '
+                               'all test files in the directory will be run.')
         self.add_argument('-v', '--verbose',
                         action='count',
                         help='Increase verbosity to include debug messages with -v, '
                             'and trace messages with -vv.')
         self.add_argument('--address',
                         help='host:port of running Gecko instance to connect to')
         self.add_argument('--device',
                         dest='device_serial',