If a will leaves money to "my children or the survivors of them", who exactly are the survivors?

If a will says that a gift is left to "my children or the survivors of them", who is meant by "the survivors of them"? If I leave a sum of money to my siblings "or the survivors of them", who exactly are those survivors?

This is a question that a reader asked recently on this blog, and I've also heard it in my office a few times lately. Since it seems to be one of those phrases that trips