From what I can tell, its a proposal to allow nodes to advertise themselves as "highly available" on the network. If a node is highly available, then the payer's software can prioritize that node for sending payments (they can also ignore the flag if they want). If the payment fails, then that "highly available" node gets more heavily penalized than a normal node would be.

Not sure how payers can penalize them though. Maybe the payer's software has a ranking of nodes to use for payments. If a highly available node fails to forward a payment, then the ranking gets more harshly demoted?