A campaign is a one-off email to some or all of your list. You decide what it says and when it goes. A new product, a restock, a sale, a seasonal gift guide, a note about what you have been making this month. Most people picture campaigns when they hear "email marketing", because they are the part you can see.
Campaigns keep your shop in front of people who are not ready to buy today. Someone who signed up in March and has not ordered yet is still reading the subject lines. When the right product turns up, they already know who you are.
How often campaigns go out
Twice a month is a sensible start for most small shops. The full answer, with the warning signs that you are sending too much, is in how often to send marketing emails. If you would rather someone else wrote and scheduled them, that is the campaigns service.