It's bookmarklet for download music from vk.com.
Sites m.vk.com and music.yandex.ru are temporary unsupported.
Just create bookmark with next url:
javascript:(function(){if (typeof initMusicDownloader === 'function') {initMusicDownloader();} else {document.body.appendChild(document.createElement('script')).src='https://mshelf.github.io/music-download-plugin/dist/downloader.js';}})();
and click on this bookmark when you on site vk.com. Any track will has new button for download.