A Shared Mailbox is a mailbox separate from your inbox, where you receive messages or choose to send messages "from" other than your personalized work email address.
Shared mailboxes are often used by multiple members of a team to manage emails to/from their customers. These emails come to/from a particular email address in one mailbox location (unlike a distribution list, which is sent directly to your inbox). Teams may also use Shared Mailboxes to send emails from a ‘generic’ email address (i.e. not from a specific person's work email address).