gradleww enables you to execute gradlew command from Gradle's subproject directory without having to specify a real gradlew path, which should be located in project's root.
Assume that you are working in a sub sub sub directory in Gradle's project. Since the gradlew script is in the top directory, you need to specify its path as below to execute it.
$ ../../../gradlew task
With gradleww (Gradle Wrapper Wrapper), you can invoke gradlew by just typing
$ gradleww task
gradleww can be installed as RubyGems package.
$ gem install gradleww
gem command should be installed as Ruby environment in most platforms. If you need to install it, see ruby-lang.org.