Root will mean that any file write as root on the client (UID 0), will be written as root on the cluster (UID O).  In general, this is a bad idea, but needed in some environments.  Read-write means that those hosts will have read-write access to the export, and the user's UID on the client will be the same for the files the write on the cluster. However, if root (UID 0) tries to write a file, it will write it as the user named "nobody" as specified in the "User/Group Mappings:" section of the web interface.
hi,    when creating an nfs export what is the main difference between root and read-write columns where i provide the names of servers?
isilon nfs export creation