Can someone explain utm parameter?

a utm parameter is just extra text on the end of a url, like ?utm_source=newsletter&utm_medium=email&utm_campaign=march, that tells GA4 where the click came from. without it the newsletter click shows up as direct or unassigned. worth it, yes, and it takes minutes. use google's campaign url builder, keep everything lowercase and consistent (email, not Email and E-mail), and only tag links that point INTO your site from outside. tagging your own internal links breaks the session into two. the two data streams don't change anything here, the tag works the same on both.