What does "mount namespace separation" in the app SuperSU do?
From the man page for mount:


  A mount namespace is the set of filesystem mounts that are visible to a process.


It makes it so that the superuser has separate mount points visible to it from the rest of the system/apps.  I believe the intention is to prevent any issues when remounting partitions, such as remounting /system as read-write.  With this option enabled it should remain read-only to unprivileged apps.

It can, however, just break things more.  It made my SD card unusable in root apps: Internal and external SD storage inaccessible as root