It means that your brother or sister had a child with you cousin.
Your grandson and your nephew are first cousins, once removed.
Your grandson and your great nephew are second cousins to each other.
You are your nephew's aunt/uncle,,, his children are your grandnieces/nephews,, his grandson is your great-grandnephew.
Their cousin, because your child is your nephew`s cousin, so your grandson or granddaughter is also your nephew`s cousin.
Their cousin, because your child is your nephew`s cousin, so your grandson or granddaughter is also your nephew`s cousin.
A nephew is a cousin to his aunt's son.
Your brother-in-law is your spouse's brother. His grandson s your spouce's great nephew. (Some people say grand nephew). That makes him your great nephew, too. Your brother-in-law could also be the husband of your sister. Then your brother-in-law's grandson is your sister's grandson and your great nephew.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
great nephew
Your father's great grandson could be your grandson, or your great nephew.
Your nephew's great grandson is your great great great nephew and you are his great great great aunt (if you are a woman) or great great great uncle (if you are a man).
Your sister is your grandson's great aunt. He is her great nephew.