The MX record preference is used when more than one MX record is entered for any single domain name that is using more than one mail server. In this case the preference number indicates the order in which the mail servers should be used. This enables the use of primary and backup mail servers.
The lower preference number is the higher priority. Two MX records with the same priority will share the workload (typically used in large ISP mail server installations). The server with the higher preference number will be contacted only if the servers with lower preference number are unavailable (this is typically used for backup mail servers).