Hello, I want to add second media widget to the backend menu with different folder. Also, i want to add description field for the media files. Is it possible?

Yes it's possible, just dynamically extend the configuration to change the folder used by the MediaLibrary using Config::set('key', 'value') before rendering the widget.

As far as adding description field for media files, there was a concept to add support for metadata for media library items (https://github.com/octobercms/october/issues/3204) a while ago, but right now probably the easiest way of achieving what you're looking for is to use a repeater with mediafinder and textarea fields.

Sorry, but i not undestand how (need to create new plugin folder - plugin.php), where put this config and how about route Backend::url('backend/media_other') ??? Can you, please, show same sample.

You can use this plugin instead then: https://octobercms.com/plugin/luketowers-filelocker.

You'll still have to find a different way to manage descriptions for media files.