A "desync" is an ExtraHop-defined term that indicates an issue with the ExtraHop appliance deployment. "Desyncs" are recorded when the ExtraHop sees a hole in the TCP transaction between 2 devices.
High desync values across your network usually means data is being lost at the monitoring interface, the SPAN aggregator, or the SPAN/network tap due to over subscription.
A couple examples:
Oversubscribed SPAN ports: A customer has only 2 100 Mbit SPAN ports available to send data to the ExtraHop. They have far more traffic than 200Mbits on their network. Any traffic over the 100 mbit threshold will be dropped. ExtraHop will not see those packets and record a desync. The network is not experiencing any actual problems, but the data being sent to the ExtraHop is incomplete. In this scenario, the span port will show some output drops
Oversubscribed SPAN aggregator: Another customer has 2 1 gig span ports feeding a span aggregator. These 2 ingress ports are the sent to the 1 gig egress port going to to the ExtraHop. It is likely that the combined 2 gig ports will send more than 1 gig combined to the ExtraHop. In this scenario, the egress port on the SPAN aggregator will show drops.
The best way to remedy these desyncs is to filter out the traffic in some way (by VLAN, protocol, application, network etc) at the location of the congestion.