Let's break it down:
* Mi: This is the Jamaican word for "I".
* Dey: This is the shortened form of "de" which means "the".
* Yah: This is a shortened form of "here".
So, the full phrase translates directly to "I am the here" which, grammatically, is not correct in standard English. However, in Jamaican, it's a common and perfectly understandable way to express "I am here."