A newsletter is a letter that contains the news. In a more sophisticated manner, an email newsletter is a type of email used in email marketing campaigns. It informs the subscribers about the latest content (news, blogs, product reviews, announcements, and other similar stuff) on your website.
Newsletters, like those created on Substack, are almost always informational in nature. The goal is not to sell something, but to provide valuable information that people are willing to pay for. For example, you might have a newsletter where you make stock and investing recommendations. If your information is good and can help others make money, then people will be more than willing to pay for it.