Welcome to the forums.
When asking this kind of question, it helps to have the context.
I take it you have been reading Patrick Kavanagh's "A Christmas Childhood".
If so, then it is a poetic phrase meaning that to a child, the light gleaming through the cracks in the barn look like little glimpses of Heaven's light streaming out of a hole in its gable [A gable is the end wall of a house under the roof - ]gable end - Google Image Search
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