The subscribers section is where you manage and segment your subscriber list. Lets take a look at the bottom page the page. There we have quick links to: Intregations, Custom unsubscribe pages and your subscriber notifications.
Higher up we have the search feature that lets you search for a specific email in all your groups. One group has already been created for you and it contains the email address that you used to set up your account.
The green profile icon will bring you to another screen where you can quickly add new subscribers. You can do this by importing a CSV or Text file, copy pasting from Excel or adding subscribers one by one.