It is the text (100 characters max) following the subject line in your inbox and it can help you boost open rates, especially on mobile devices.
On smartphones the preheader is usually shown on the third line below the subject line n your inbox.
Here's an example of how the preheader looks on your smartphone:
The preheader text is found in 2 different areas:
To find out more about the preheader you can watch the video below