The "Priority" value or number for some DND records such as MX Records is used to indicate which of the servers listed as MX records it should attempt to use first.
Example of how Priority works
If you are using two MX records that have separate priority values and point to different subdomains. The MX record with the lower priority number (“0” in this case) is the first to be tried for email delivery. If this server is unable to handle the mail request, the next lowest priority number is used, which in this case would be 10.Some email providers have only one MX record and some have well over two.