This is just an extra information injected in PeerTube HTML that is required by Twitter/X. If you don't have a Twitter/X account, just leave the default value.
Put your instance on the public PeerTube index: <x id="START_LINK" ctype="x-a" equiv-text="<a href="https://instances.joinpeertube.org/instances">"/>https://instances.joinpeertube.org/instances<x id="CLOSE_LINK" ctype="x-a" equiv-text="</a>"/>
Moderators of <x id="INTERPOLATION" equiv-text="{{ instanceName }}"/> will have to approve your registration request once you have finished to fill the form.