Can someone explain me this irule

The LB_FAILED event is triggered "when LTM is ready to send the request to a pool member and one hasn’t been chosen (the system failed to select a pool or a pool member), is unreachable (when no route to the target exists), has reached a queue limit, or is non-responsive (fails to respond to a connection request)"

https://devcentral.f5.com/wiki/iRules.LB_FAILED.ashx

So this basically says that when this event is triggered, and an active members check of the specified pool indicates that no pool members are available, then send the traffic to the specified node on port 8100 (presumably not one of the pool members). Ironically, if the LB_FAILED event is triggered, there's a good chance that the active members check will also return 0 (if the check is for the pool applied to the VIP), so the two are somewhat redundant. You normally find the active_members Boolean command in other events like CLIENT_ACCEPTED or HTTP_REQUEST, where you can make this pool member determination BEFORE an LB failure occurs.