Switch is a network device used to connect IT devices. It can be considered a multiport ethernet bridge.
Switch works on the data link layer. It has a MAC address table that associates MAC addresses with ports. It also contains TTL (time to live) that sets the maximum time of the period of inactivity, after which the entry will be erased.
At the initial switch launch the table is empty, it gets filled in during a learning mode. The ports are filled in with sender's MAC addresses. If the MAC address is not associated with any of the ports, it will be forwarded to all of them.
There can be multiple MAC addresses associated with one port.