In this field you can enter the actual review.
You can use the rich text editor for rich text formating. You can also enter HTML to embed objects, e.g. to embed a YouTube video or a screenshot of the project. In this case use '[' and ']' to mark the passthrough HTML as such.
Please note that the first time you use the new UI your description is converted from rich text to MIME. You might want to copy and paste the raw plain text from the old UI in the new UI so that you don't loose information.