A Space is what the web-based 'TiddlySpace' provides to host your Wiki.

Read about TiddlySpace to understand the broader picture.
A Space can belong to a single member, or that member can make other people members (see Making someone a member of a space).
If there is more than one person as members of a particular Space this is what the TiddlySpace developers call a 'small trusted group' - all of whom have equal rights (just as giving someone a key to the door of the office or your home would entail some trust.)

Spaces can include other spaces!

What is neat about TiddlySpace is that one Space can include OTHER spaces (that is, the Public content from wikis that are curated by other people.)
These included Spaces might add content, themes (which determine how the wiki looks and behaves), or extra functions.
In the Index, one of the Tabs you will see is Spaces, adn this lists all the Spaces that are INCLUDED in the present one to give you the nd reulst you are looking at.
If you are an advanced user, you may want to know about Including spaces to add further content/functions to your own wiki.