You can google for a description of "/etc/exports" file, e.g. here:http://tldp.org/HOWTO/NFS-HOWTO/server.html
IP address is the address of the device (i.e. the WD) that should be granted access
Root_squash is to... 'squash' access as root, i.e. can root@WD have full read/write access to
the NFS share? (as all root user accounts usually have). If you create a specific user on Synology to access NFS, I don't think you need that.
Asynchronous: as you can read from the document I linked, it may be worth to disable it in case you have a client that writes to the share, for safety reasons. WD Live at 99% is only reading so I think you can leave it enabled.