What is a Space?
This feature was newly introduced in Kibana 6.5.0. Kibana Spaces allows you to organize your dashboards, visualizations and other saved objects into meaningful categories based on team, use case, individual or really any way you choose to use it. The beauty of this feature is each space created is independent, so objects that exist in one space don't clutter other spaces.
Spaces with Security
Spaces will be available as part of our default distribution as an organization feature, and will be securable once Security is enabled. Using RBAC you can layer in security to control who can (and cannot) view and/or edit which space. When a user logs into Kibana, they will be presented with a space chooser that allows them to choose a space. From this point forward, all saved objects will be filtered and saved within that space. If you have a shared Kibana instance with 100s of dashboards and visualizations, spaces allows you to deliver a more organized and secured experience to the end users. When users log into Kibana, if they have access to more than one Space, then they see the Space Selector screen. Security limits which spaces users have access to, so if they only have access to one space, then they won't see the Space Selector screen.
When a space is deleted, it will cascade the delete to all saved objects within the workspace. Additionally, Kibana UI Settings will be space specific, and the settings from the default workspace will be copied to the new space when it's created.
Very interesting, isn't it? 
Want to learn more? Hop on to the awesome blog post penned by our very own Spaces developer @Larry_Gregory which dives right into Spaces here. It also covers the security aspect of it.
Here are some eye candy screenshots which show the awesome spaces I created ( you can see the Default space when you login and the whole list of spaces created too):
Different Spaces created:
Did you know that you easily could move the saved objects between spaces? Dive in right here to know more how this can be achieved via import/export. The Migrating to Spaces blog will teach you everything that you need to know.
Additionally there is more documentation on Kibana Spaces API.
(This API is experimental and may be changed or removed completely in a future release. The underlying Spaces concepts are stable, but the APIs for managing Spaces are currently experimental.)