What is Synology Hybrid RAID? - SynologyWiki:
What is Synology Hybrid RAID (SHR)?
The SHR is an automated RAID Management system, designed to meet the needs of easy deployment of a storage volume. The SHR is designed to meet the needs for those users who do not care or do not want to know what are the finer aspects of the various RAID-levels which are currently supported on the DiskStation, and would like to use the SHR to handle RAID management, storage expansion and maximize storage capability. The SHR allows for 1-disk worth of redundancy - meaning that the SHR volume can suffer the lost of one disk, and the data volume will still be available for use. If two or more disks fail within an SHR volume, then the data volume will be lost. Note that a RAID volume (whether classic RAID or SHR) is not a backup system, please refer to the What is a Backup? article for further information.