Storage Spaces Direct is a feature introduced in Windows Server 2016 that extends the software-defined storage (SDS) stack in Windows Server to enable building highly-available (HA) storage systems with local storage.
Microsoft developed Storage Spaces Direct, or S2D, to help simplify the deployment and management of software-defined storage systems as well as to open the use of new classes of disk devices like SATA and NVMe (Non-Volatile Memory Express disks) disk devices, which were previously not able to be used with clustered Storage Spaces with shared disks.
Storage Spaces Direct in Use
Storage Spaces Direct can use disks internal to nodes in a cluster or it can directly attach to nodes in a cluster via an enclosure. Storage Spaces aggregates the disks local to the cluster nodes, which creates a Storage Pool of these disks and enables virtual disks to be created.
These virtual disks can be used as a Cluster Shared Volume (CSV) that can house Hyper-V virtual machines (VMs) running on ReFS-formatted volumes.
What this essentially means is that with Storage Spaces Direct, HA storage systems can be built using storage nodes with only local storage either with disk devices that are internal to each storage node or disk devices in JBODs (Just a Bunch Of Disks) where each JBOD is only connected to a single storage node.