Spaces allow you to organize your work in Ossum. A space contains a board with columns that define the states in your process and work items or cards for the things you need to work on. When you create your Ossum account an initial space is created for you and if your plan allows it, you can create additional spaces.
Whether you use just one space or many spaces largely depends on how you want to organize your work as the space largely exists to separate your work items.
Control which users have access to this space.
The relationship between spaces and repositories is more of an "association" then a tight 1:1 coupling. A single repository might contain code that is related to work in multiple spaces. From this tab you can create new repositories as well as link existing repositories with this space. The space serves as a convenient filtering mechanism when working with repositories by letting you just see the repositories, merge requests and code that is associated with the space you are working in.