Parenting in op5 Monitor is used to determine whether a host is down or unreachable.
A host is...
down if the host is the first one it can not reach in the "tree"
unreachable if the host is after the host described above.
This example describes how the parenting works in practice
The picture below shows how a network looks like from the monitor servers point of view.
As you can se everything starts with the op5-monitor server. If fw-01 is down, as shown in the picture above, all child hosts of fw-01 is considered as unreachable.
The example above shows that you can use parenting to exclude a lot of unnecessary alerts and notifications. This because you can tell op5 Monitor not to send any notifications for a host that is unreachable. That means you will only get notification about fw-01 in this case, not the hosts "below" fw-01.