Preheader
The line of text an inbox shows next to the subject line, before the email is opened.
A preheader is the snippet of text that appears after the subject line in the inbox list. Mail clients take it from the very start of the email body, which is why an email that begins with “View in browser” shows exactly that next to its subject.
Because it is pulled from the body rather than set as a header, you control it by putting a short hidden line at the top of the email. Most builders, including Blocks, give it its own field so you do not have to hide anything by hand.
Treat it as a second subject line: it has roughly the same length before truncation and it is read at the same moment. Repeating the subject wastes it; continuing the thought does not.