"In their entirety", logically, means "not only in part", and must mean that no part of a particular share or shares can be paid to the intended beneficiary.

"Fail", applied to a whole will or to a particular bequest or bequests in a will, means being unable to be put into effect. In the case of a whole will this will usually be because the will was not properly executed (there are many possible reasons why this might be the case). In the case of a particular bequest, including, as in this case, a share in the residuary estate, the most common reason will be because the beneficiary predeceases the testator. There are other possible reasons. There could be a fault in the way the bequest is formulated in the will, for example; the beneficiary may not be properly identified. Another possible reason is that the beneficiary in question, or his or her spouse, acts as a witness of the will; this is not allowed (at least in the UK, under the Wills Act 1837). Further examples of failure of bequests can be specific bequests, when a specific piece of property is bequeathed but turns out to have been sold before the testator dies, and contingent or conditional bequests, where the beneficiary has failed to fulfil the condition.

The point of including this provision is that the desired result may not happen by default; if one of the residuary bequests fails, that share may not automatically be divided among the remaining residuary beneficiaries. In the absence of further provision, the result will be partial intestacy in respect of that share, which will be dealt with according to intestacy laws. This may not produce the desired result.

For further information, see, for example:
http://www.rainulegalconsultants.com/why-some-wills-fail-2/
http://www.makingawill.org.uk/text-types-of-gift-page-134.ht...
https://books.google.es/books?id=Bj6P_lFwe-gC&pg=PA75&lpg=PA...
http://thechildrensfoundation.ca/?page_id=172