what does this ** symbol mean in gulp?

Question

This might be a beginner question, but I want to understand what does the ** mean in the following context:

    gulp.task('images', () => {
  return gulp.src('app/images/**/*')
    .pipe($.cache($.imagemin()))
    .pipe(gulp.dest('dist/images'));
});

Does it mean grab anything that's inside the images folder? why does it use ** instead of a single *? is there a difference in meaning?