UTM tags are labels in a link that help determine where a visitor came from to your site.
Typically, these labels are added manually. Some advertising systems set them automatically when someone clicks on an advertisement. However, if a visitor comes to your site not from an advertisement, such labels usually aren't present.
Imagine that you ordered a promotional banner on SITE.COM. If the banner features a simple link like "https://mysite.com", no additional data will come to your site when someone clicks on it. For your site to recognize that the visitor came from SITE.COM, the link should look like this: "https://mysite.com/?utm_source=site.com". This way, your site will see this tag and understand that the visitor arrived from SITE.COM.