Read the man page for "nfs"
The quote is an excerpt from it:
bg/fg Determines how the mount(8) command behaves if an attempt to mount an export fails. The fg option causes mount(8) to exit with an error status if any part of the mount request times out or fails outright. This is called a "foreground" mount, and is the default behavior if neither the fg nor bg mount option is specified. If the bg option is specified, a timeout or failure causes the mount(8) command to fork a child which continues to attempt to
mount the export. The parent immediately returns with a zero exit code. This is known as a "background" mount. If the local mount point directory is missing, the mount(8) command acts as if the mount request timed out. This permits nested NFS mounts specified in /etc/fstab to proceed in any order during system initialization, even if some NFS servers are not yet available. Alternatively these issues can be addressed using an automounter (refer to automount(8) for details).
There is a lot
more there; it would be wise to read it.
In case you'r not familiar with how to access the man page, the command is
hostname:~ # man nfs