A unicast is a message being sent to a single destination. So what is unicast flooding then?
When a switch receives a frame destined to a host in which it does not have in it’s CAM table then the switch will flood the unicast frame to all ports with the same VLAN ID.
Basically, the unicast becomes a broadcast. The term for this is unknown unicast flooding.
In the example below, let’s assume Host A sends a frame to the switch destined to Host B. Because host B is not in the CAM table, the switch will flood the unicast frame to all ports with the same VLAN ID. This is what the CAM table looks like for the switch as Host A sends it’s frame:
[table id=3 /]