The “Preheader” or “Johnson Box” as it’s sometimes called appears in the list view of many email clients. 

It follows on directly after the subject line and needs to work in conjunction with it. 

It can also work as a standfirst (or subheading or whatever you’d like to call it) due to it’s relative prominence on the email. 

Half-decent online articles about preheaders here:

Last updated on 15th February 2019