The Mailer Daemon is the software on a mail server which handles emails. You would usually see messages from it if, for example, an email you send could not be delivered immediately or couldn't be delivered at all because, for example, you mistyped something in the email address.
If you're seeing a lot of them, and they are not emails you have sent, your email address is being used as the from address for a load of spam emails, most of which will be undeliverable. They will probably stop in a couple of weeks when the spammers switch to another from address.