A calendar feed is a link you hand to another app, or another person, so they can see when you are busy. Until now you had exactly one, covering everything you had connected, so the link you gave a client showed the same events as the one you subscribed to yourself.
Feeds are now named and you can keep as many as you need. Each covers the calendars you pick for it and carries its own event name settings, so a feed built for a client shows nothing but busy blocks while the one you use yourself keeps the real titles. Every feed gets its own page and its own link, in a field you can select and copy.
Feed links are not derived from anything about you — the token is generated without taking any input from your account — so knowing your email tells nobody the address of your feed. Free accounts keep one feed; keeping several is part of a paid plan.
Read more about what a paid plan adds.