Email archiving is an automated process for preserving and protecting all inbound and outbound email messages (as well as attachments and metadata) so they can be accessed later. In other words, email archiving is storing emails and making them searchable.
Email archiving providers take this burden off organizations by storing emails on their servers while making them accessible to designated administrators in the organization. This is different than simply creating an email data backup. Data backups do not allow searching, so if a particular email needs to be found, it might take weeks for you to find it.