What is a site-wide role?

A site-wide role grants access across projects, optionally inclusive of administrative permissions.

As a site administrator, you can create and manage site-wide and global roles.

Site-wide roles enable you to create restricted site administrators who can assist you in site administration tasks. At the same time, without providing administration access, you can set across site tool/application access for any user.
Global roles enable you to provide ready-to-use roles that the project administrators can use while assigning tasks to their project members. For example, you can create a "Developer" ready-to-use role and provide access to tracker, documents and source code tools. Project administrators across the site can assign this role to all the developers in their projects to set their relevant permissions.