Controls which hosts the server will respect the root user on. If a client is not on the list, even if it can mount the share read-only or read-write the NFS server will treat the root user on client as the Anonymous User.
Hosts can be specified in the following ways :

Hostname or IP address (like ftp.foo.com or 1.2.3.4)
  Matches the host with the given name or address
Netgroup (like engineering)
  Matches any host that is a member of the netgroup
DNS domain (like .foo.com)
  Matches any host in the domain
Network (like @10.254.1)
  Matches any host in the network
Network/netmask (like @10.254.1/24)
  Matches any host in the network

In addition, any of the above notations can be prefixed by a -, meaning that the host, netgroup, domain or network is not allowed to mount this share. This can be useful for allowing access by a set of hosts, but denying one member.