A UTM parameter is one of five specific query parameters that are used to convey information about where the user came from. These five parameters are:
Originally UTM stood for "Urchin Tracking Module" and was a key part of the web analytics tool "Urchin Analytics". Google, taking interest in this tool, did what Google does and acquired Urchin Analytics. Shortly thereafter Google rebranded their acquisition and gave it the name we're familiar with today; Universal Analytics. Despite this, UTM parameters have found their way into almost every digital marketing platform out there today and play a key role in helping digital marketers identify where a user came from.
Here's a great post from our friends over at Triple Whale that dives in deep to UTM parameters and gives some more context on how to use them.