man exports
...
Very often, it is not desirable that the root user on a client machine is
also treated as root when accessing files on the NFS server. To this end, uid
0 is normally mapped to a different id: the so-called anonymous or nobody
uid. This mode of operation (called `root squashing') is the default, and can
be turned off with no_root_squash.
By default, exportfs chooses a uid and gid of 65534 for squashed access.
These values can also be overridden by the anonuid and anongid options.
Finally, you can map all user requests to the anonymous uid by specifying the
all_squash option.