Alt text
The words shown in place of an image that has not loaded — or read aloud instead of it.
Many clients block images until the reader allows them, so a share of your audience meets the email as text and empty boxes. Alt text is what stands in those boxes, and what a screen reader announces.
This matters more in email than on the web, because email is more image-heavy and image blocking is more common. An email whose entire message lives inside one picture says nothing at all to those readers.
Describe the purpose, not the file: “Spring sale, 30% off” rather than “banner-1.png”. Purely decorative images take an empty alt so they are skipped instead of announced.