URL parameters that track where a click came from. The plumbing of attribution.
UTMs (Urchin Tracking Module tags) are query parameters on URLs: ?utm_source=facebook&utm_medium=cpc&utm_campaign=winter-promo. They let your analytics tool (Google Analytics, whatever) know where each visitor came from. Without UTMs, all paid traffic looks like 'referral from facebook.com' and you can't tell which campaign drove what.
yoursite.com/?utm_source=linkedin&utm_medium=paid&utm_campaign=q4-enterprise. When someone clicks, your analytics knows 'this visit came from the q4-enterprise LinkedIn campaign.' Now ROAS, CPA, etc. can be computed per campaign.Consistent UTM hygiene is non-negotiable for data-driven marketing. Every link in every campaign, every email, every social post, should have UTMs. Pick a naming convention and enforce it. Without UTMs, your attribution data is garbage.