What is a preheader (or preview text)?

The preheader (or preview text) is a short line of text that is shown in the email client before the user opens an email. The preview text is separate from the subject line. 

The preview text is only displayed in certain email clients:

ProviderPreheader shown?Correct display in newsletter?
gmail.com
YesYes
yahoo
YesYes
AOLNoYes
freenetNoYes
outlook.comNoYes
t-onlineYesYes
thunderbirdNoYes
AnroidYesYes
iOSYesYes

Learn more about preheaders here