A space is a separate library with its own folders and captures. Everyone starts with a personal space, and the space switcher in the toolbar moves between them or creates a new one with New space.
How many personal spaces you can own depends on your plan, and the pricing page lists the number for each.
Sharing
Sharing a folder turns it into a shared space, and the people you invite by email see everything inside it. Spaces other people have shared with you appear in the switcher under Shared with you.
To show one saved page to someone without an account, use a review link instead. See Review links.
An invitee is either an editor, who can add and change things, or a viewer, who can only look. Shared spaces belong to team plans, and the pricing page shows when those are available.
If your plan changes
Moving to a plan with fewer spaces never deletes one. Your default space stays writable, or the one you used most recently if you have none set, and the others become view-only with everything in them intact. Moving back to a plan that covers them makes them writable again, with nothing for you to do.