What twclid actually is
twclid, the X Click ID (the name dates back to when X was Twitter), is appended automatically when someone clicks a paid ad on X. It is per-click, generated fresh for that interaction, and its job is the same as every other click ID on this list: let the destination site tell X's Conversions API "the visit tied to this exact value led to a signup, a purchase, whatever the advertiser is measuring", closing the loop between ad spend and outcome.
Ads only, not organic links
This is the detail worth holding onto: a link someone pastes from an ordinary post, reply or profile does not carry twclid. It only appears when the click came through X's ad delivery system. If you see twclid on a URL, the visitor arrived via a paid placement, not word of mouth, which makes it a useful signal in your own analytics for telling paid X traffic apart from organic X traffic without needing a hand-typed UTM tag to do it.
Is it safe to remove?
The page works exactly the same for you without it. For the advertiser, X's own matching guidance treats twclid as the strongest available signal for tying a conversion back to a real account on the platform: at least one identifier, typically the click ID, an email address or a phone number, has to be present for a conversion event to match properly. Strip twclid from a link before the conversion fires and that match gets weaker, not impossible, since email or phone hashes can substitute, but harder.