An MX record has a FQDN and a priority. The priority is a number which is used to choose which mail server to use if multiple MX records exist for a domain name. A mail server trying to send an email to you will always try the lowest number priority first!

Additionally an MX record has a host name to allow you to use e-mail addresses with subdomains. The hostname is usually left blank to specify the mail server for the domain name on its own.

If you have a mail server at mail.someserver.be and you want to add this for your domain name yourdomain.be, you would add an MX record with a blank hostname, an FQDN of mail.someserver.be, and a priority, for example 10.
This would mean that mails sent to @yourdomain.be will be delivered at the mailserver mail.someserver.be.

If you had a backup mail server that you only wanted mail delivered to in case of a problem with your main mail server, you would add another MX record with an FQDN of mail.someotherserver.be and a higher priority number, for example 20 or 50.

This would mean that any mails addressed to @yourdomain.be which can not be delivered to the server at mail.someserver.be will be delivered to the server at mail.someotherserver.be.

Adding a hostname into the MX record simply means that you are specifying the mail servers for a subdomain. For example adding an MX record for hostname "sales", FQDN mail.salesserver.be with priority 10 would mean that e-mails sent to @sales.yourdomain.be will be delivered to the mail server mail.salesserver.be. E-mails addressed at @yourdomain.be will not be affected.