What UTM tags do
UTM parameters tell Google Analytics where a click came from. A link posted on Facebook and the same link sent by email look identical without them. Source is the site or platform, medium is the channel type, and campaign is the specific push you're tracking.
Keep it consistent
Lowercase everything and use underscores. "Facebook", "facebook", and "FB" become three different sources in your reports. This tool lowercases and replaces spaces for you.