Hiraeth is a Welsh word which refers to homesickness, mixed with grief and sadness, over the lost or departed; a type of longing for ones homeland, or the romanticized past. It calls to memory a home that once was, or one which may never have actually existed. It represents a mixture of nostalgia, wistfulness, and yearning.
This song is dedicated to the memory of those who left their homes to go to war, and those who remained; those who returned, and those who did not.
And it's for those who left a beloved home, in order to create another.