A DNS MX Record (or Mail Exchange record) directs email to the correct server for a particular domain. If a domain has more than one mail server (for resilience against failure) more than one MX Record can be present.
If multiple MX Records are present, mail will be attempted to be delivered in priority of the order of listing e.g. if mail can't be delivered using the first priority MX Record, the second priority MX Record is used, and so on.