What “Yardie” means
A yardie is a Jamaican, someone from "yard". The word comes from yaad (yard), which means home and, especially to Jamaicans abroad, Jamaica itself.
Examples
Mi a yardie, born an grow.
I'm Jamaican, born and raised.
How to use it
Many Jamaicans, at home and abroad, use it proudly. Context matters: in the UK in the 1980s and 1990s the press used "Yardie" for Jamaican gangs, so outsiders using it can come across badly.
Where it comes from
From yaad (yard), meaning home and, by extension, Jamaica.