The FlexVPN Load Balancer feature provides the ability to cluster a number of FlexVPN gateways and distributes incoming IKEv2 connection requests evenly among each of these gateways. Using IKEv2 redirects the incoming client requests are redirected to the least loaded FlexVPN gateway based on the system and cryptographic load. This allows for a scalable pay-as-you-grow architecture where VPN gateways can be added or removed from the cluster, depending on the number of sessions required.
The FlexVPN load balancer is constructed of a number of components. By understanding those components, you will have a great understanding of the overarching feature.