A switch populates its mac-address table with mac addresses registered on incoming frames. As a result, when the switch needs to forward a frame destined to that specific mac-address, it will know out of which port to send the frame. Flooding however occurs when the switch does not know of the destination mac-address ?say the switch has not learnt that mac address yet; or maybe that specific entry expired so it got flushed away from the mac-address table. To ensure the frame reaches its intended destination, the switch will replicate that frame out of all ports, less the port where the frame was received that's flooding.
http://blogbt.net/index.php/2015/03/mac-address-table-arp-table-and-unicast-flooding-part-i/