Direct Answer
utm_source is the platform sending traffic (google, facebook, newsletter). utm_medium is the channel type (cpc, organic, email, social). utm_campaign is the specific campaign or initiative (spring-sale-2026). All three together let GA4 segment performance precisely. Add utm_content for ad/link variant and utm_term for keyword.
Short version
“UTM source is the platform like google or facebook. Medium is the channel type like cpc or email. Campaign is the specific initiative.”
Use lowercase, hyphen-separated values. Mix-cased and inconsistent values break segmentation in GA4.
Don't tag organic Google or direct traffic with UTMs — Google's auto-tagging handles those.