MX ("Mail eXchange") records are used to determine the delivery route for e-mail. For example, if you want to host your own mail server for the domain in your account, you will need to specify the mail server names as MX records.
In addition, you can specify multiple mail servers and rank them by priority in the event the primary mail server is not responding. If you give the same rank to different mail servers, the traffic load to those servers will be shared. E-mail will be delivered to the server with the lowest priority number.