What are UTM URLs?
UTM URLs let you track visitors to your marketing campaigns in Google Analytics. By adding certain parameters to the end of your URLs, you can attribute visits to those URLs to particular marketing channels and campaigns.
For example, let's say you're sending an email to all your customers. To see how well the email does - and see how many people click through to your website - you'd add UTM parameters to the URLs in your email.
A UTM URL for a campaign like this might look like this:
https://yourwebsite.com/your-page/?utm_medium=email&utm_source=subscribed-customers&utm_campaign=winter-sale
What does UTM stand for?
Before it was called Google Analytics, the popular web analytics platform was called Urchin (before Google bought it and rebranded it).
UTM simply stands for Urchin Tracking Module.
Bear in mind...
UTM parameters are only going to be any good if you have Google Analytics running on your website. And they're only good for tracking visits to your own website, so don't bother adding UTM parameters to links that go to other people's websites (unless they've asked you to include them for their tracking).