MAC address or Media Access control address is a 48-bit/6-byte address associate with a network interface card on a host or a network interface on a router/switch. Mostly, MAC address is permanent on the hardware. So sometimes it is called physical address or burn-in-address.
MAC address consists of 3-Byte of organization identifier and 3-byte of Network Interface Identifier.
On switch, MAC address table is used to associate MAC address, VLAN to specific physical port so that Ethernet packet could be correctly forwarded by MAC address look-up.
MAC address table could be configured statically or learned dynamically. The entry will age out because of
MAC address table is a rare hardware resource and the number of entries are limited.
On host, MAC address is used to identify the specific physical network interface on the LAN network.
For further details, see MAC address wiki page.
To understand how MAC address works in switch, read this article.