If you've ever used bookmarklets in a "normal" web browser it's exactly the same:

1. Add the bookmark to your bookmarks (by tapping and holding on the link on the VidConvert page
2. Go to a YouTube page.
3. Click on the newly added bookmark and the current YouTube video will be automatically submitted to the VidConvert queue.

This is supposed to be easier than bringing up the URL bar, copying the YouTube URL, going to VidConvert and pasting it.

HTH,

Andrew