It is two things, and people usually only think of the first one. The first is campaigns: the emails you decide to send, to some or all of your list, on a day you choose. A new product, a restock, a sale, a note about what you have been making. The second is automation: emails that send themselves when somebody does something, like signing up, leaving a basket or buying for the first time.
Campaigns keep you in front of people. Automation catches money that would otherwise walk out. Most shops start with campaigns because they are obvious, and most of the earnings sit in the automation they have not built yet.
The part nobody mentions: it is the only marketing channel where the audience is yours. Ads stop when you stop paying. Social reach depends on whoever is running the feed that month. The list is a file you own, and it keeps working. That argument is set out properly in email versus social media.
The longer version, with the flows most shops start with, is on what email marketing is.