BS"D is an English transliteration of the Aramaic abbreviation of b'siyata d'shemaya; literally "with the help of Heaven," an idom for "with G-d's help."

It is a custom practised by many Jews to write this at top of documents, to appeal to G-d with the knowledge that if the correspondence contains any errors, it is only because of human fallibility. Anything pertaining to holiness should not be judged by the personal, human error of the composer.