If you right-click an item in the Media Library > Edit selected items, and you check the option 'Update file tag(s) if supported', then yes, the ID3-tags of the mp3 file on your hard drive will be updated. (screenshot
Note the 'supported'. That means that if you edit info of a wave or avi file (for example), the info will only be stored in the library database and not in the actual media file on your hard drive.
You can check this by right-clicking an item in the Media Library > View file info...
The info displayed there is always stored in the ID3-tags of the actual media file.