Does anyone know who or what mailer-daemon@googlemail.com are as they keep sending me emails saying

This is an automatically generated Delivery Status Notification

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

Delivery to the following recipient has been delayed:

I have been emailing the same person for a number of months and a few days ago this came up, if anyone can help I will be very grateful

Probably a problem with whatever email service they are using, which is (temporarily) stopping them from receiving emails.

Maybe their inbox is full?
Mailer daemon is just a program on the server or wherever that sends out automatic responses when a mail can't be delivered. (Or something like that!)