On mobile devices and some email clients there is a short line of text that is displayed either below or to the right of the email subject line. This preview text (also known as a preheader) is displayed to provide the recipient with a short summary of what the email contains.
If you do not include preview text, any email clients that display preview text will just pull a snippet of the first line(s) of text in the email. So, adding preview text gives you control over what your recipients see and is another way to capture their attention and further entice them to open your email.
A good subject line and preview text combination could mean the difference between someone opening your email or sending it to the trash. This is especially important for mobile since more emails are opened on mobile devices than on desktops. The amount of preview text that is displayed varies between email clients and apps, so it’s best to front load your preview text with relevant keywords and phrases.