Simple definition
A Linux user namespace maps user and group identifiers separately from the host namespace.
Purpose and use
A user namespace gives processes a separate view of user and group identifiers and selected privileges.
How does it work?
Mappings connect internal identities with identities in a parent namespace.
Example and verification
For an unprivileged container, inspect UID/GID mappings when a mounted volume has ownership problems.
Points to watch
Being root inside a namespace does not mean holding every host-root privilege. Security also depends on the kernel and other restrictions.
In short
Interpret root identity in its namespace and capability context.