UTM parameters are tags added to the end of a link, such as ?utm_source=newsletter, that tell analytics tools where a visitor came from.
The common ones are utm_source (the site), utm_medium (the type, like email or social) and utm_campaign (the campaign name).
They don't change the page; they only label the visit so you can compare sources.
Why it matters for a launch
Tagging the links you post during a launch shows which platform or post actually brought users, not just visits.