A calendar feed is a link you give 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 has its own event name settings, so a feed for a client shows only busy blocks while the one you use yourself keeps the real titles. Every feed has its own page and its own link, in a field you can select and copy.
Free accounts keep one feed, and keeping several is part of Pro.
Shareable calendar links covers what a feed carries and how many you get on each plan.