A mailing list is a type of ongoing conversation via email. When you subscribe to one you are added to a list of email addresses. When anyone sends mail to the list-address the mail is forwarded to all the addresses on the list.
Most mailing lists are unmoderated, so there are few restrictions on what can be posted.
Moderated lists may require all messages to be vetted by the list owner. Mailing lists have an administrative address distinct from the address to which conversational mail is sent. Typically, the administrative address is automated, and managed by some form of list manager software, or 'listserver'. This means that mail sent to this address is dealt with by a piece of software, not a person. You send commands to control your participation in the list.