A UTM code is a snippet of simple code that you can add to the end of a URL to track the performance of campaigns and content in Google Analytics.
A UTM code looks a little like this:
What can you track with a UTM link?
1. Traffic Source
Source allows you to track where the traffic originated from. For example if the traffic came from Facebook or Twitter.
'Medium' tracks what type of advert the visitor came from - for example was it an organic Facebook post or a paid advert?
3. Campaign name
The campaign name allows you to track the performance of a specific campaign. For example, you can use this to tell the differentiate between different Facebook posts.
There's plenty more you can use UTM's for, here's a useful link if you want to read a little more.