For example, if you built a public park, you might make a path to walk on, a picnic area, a playground, some bathrooms, and a soccer field. These areas were made with tasks in mind.
If parkgoers wear down a path through your fresh laid grass, you as the parkitect (ha!) could see it as an annoyance. Or you could see it as a space between places and pave over it so people can get where they want to go without walking through the mud.
A space is an open, free, or unoccupied area.
No matter what you're making, your users will find spaces between places. They bring their own context and channels with them, and they show you where you should go next. Find areas in flux and shine a light on them.