RAID 1 is a hard disk configuration where the contents from one hard disk are mirrored onto another. This provides the user with some redundancy in case a disk fails. On your Linux system, the two hard drives are represented as a single file system. But in the background, making changes to your files is actually writing the changes to two disks at the same time.
This is a companion discussion topic for the original entry at https://linuxconfig.org/linux-software-raid-1-setup