After reboot, the target server is trying to mount the nfs filesystem as part of boot process, in the same time the source server went to offline or some one reboot the source server, in this case what will happen.
If the file system mounted to the target server as soft mount, then it will contact the source server and trying to mount the filesystem, if it does not get the source server within a while, then it will stop to contact the source server and make the target server to come online without mounting the file system.
If the filesystem mounted to the target server as hard mount, then it will wait until the source server to come online to mount the filesystem as per the above scenario, so in this case the target server can not up until the source server is accessible.
To put in simple words:
A soft mount will return an error if the source server goes offline.
A hard mount will wait until the server comes back online, and then begin to transferring /copying again.
It is really best practice when you mount the filesystem as soft mount.