A subscriber ID in plain sight
Kit's help center describes a setting that will "automatically append a ?ck_subscriber_id=123 parameter to all the links sent via Broadcast and Sequence emails, where '123' will be replaced with the subscriber's unique ID." Broadcasts are one-off newsletters. Sequences are automated email series. The number on the link is the ID of the subscriber the email was sent to.
That sets it apart from most email tracking parameters. Klaviyo's _kx and HubSpot's _hsenc are encoded strings. ck_subscriber_id is the subscriber ID itself, readable as it is.
The sender switches it on
The parameter comes from a toggle in Kit's advanced settings labeled "Add subscriber_id parameter in email links." The account owner turns it on and saves the change, and from then on the ID is added to links in broadcasts and sequences.
What reads it
Kit names two uses. On a WordPress site running Kit's plugin, the ID lets the creator track subscribers who clicked through from an email, tag subscribers who visit specific pages, and show different content based on a subscriber's tags. For Creator Pro accounts, it feeds on-site activity into Kit's Insights dashboard.
Strip it before you share
Deleting it does not change the page. For the creator, a stripped link means that visit is not tied to a subscriber, so page-visit tags and tag-based content do not apply to it. For you, a copied link hands your subscriber ID on that creator's list to whoever opens it, and the tagging above is based on that ID. Post the version the decoder above gives you, with the ID removed. Newsletters on other platforms carry their own versions, such as mc_eid on Mailchimp.