gradlew is an executable file created by the gradle wrapper plugin, to replace the normal gradle executable for a project
While the normal Gradle executable comes with the installed version and therefor depends on that version, gradlew downloads the correct version of Gradle (specified in the Gradle build script) and thereby ensures that the build is independent of the installed Gradle version.
One obtains a gradlew executable by applying the Gradle wrapper plugin in the build script, specifying a Gradle version and executing
gradle wrapper
After that gradlew can be used whenever one normally would use Gradle.
Official Gradle website: https://gradle.org/
