To add to what rccpgm said, if you look at the mase definitions, that will help clarify more.
Unicast are individual streams of traffic sent to a single host address. When the frame is sent to to switch, if the switch has no knowledge of that host (recorded in the bridging table), then you have an unknown unicast.
A juniper summary can be found here: https://www.juniper.net/documentation/en_US/junos14.2/topics/concept/rate-limiting-unknown-unicast-forwarding-understanding.html