"Letters" here ought more properly to be translated as "document", but as it seems traditional to do so, we shall leave it as it is. This kind of document is a certificate from a bishop, meant for other bishops and church authorities, that the bearer is a person of good character and with all the qualities and skills needed to be a priest or to serve in other capacities. The uses of this document have been changed in the reformed Code of Canon Law of 1917, so the meaning of the sentence is different according to whether it was written before or after 1917. The two web references I attach refer respectively to the modern meaning (wikipedia) and to the pre-1917 one (Catholic Encyclopedia).