You can add preheader content in the message editor.

It's an extension to your subject line. They complement each other with the goal of getting your email opened. (more about subject lines here)

Besides the sender name (the "from name") and the subject line the preheader text is the third key piece of information that influences open rates.

When people receive emails they want to know "Why should I open this email?"
The subject line and the preheader text should answer this question. 

The preheader shows up in inboxes next to or below the subject line. It's not part of the email and it does not show up in the message once it's opened. Just like the subject line.

Unlike the subject line, preheader text isn't required to send emails - but it's highly recommended. 

Without a preheader sentence the first piece of text from your email is displayed in inboxes after the subject line. This is often some non-interesting text, such as a “view online” link, a date or salutation. This is not a great way to tease your readers to open and read your message. 

See the example below of an email in an inbox with and without a preheader text:

Email clients generally display at minimum of 40 characters on the low end and up to 100 in some cases. That's a huge range! It depends on individual inbox settings and email client defaults. 

In Envoke the preheader cannot be longer than 75 characters but it's best to ensure that your preheader content makes sense even if it's cut off after 40 characters.