This was often used in conjunction with a signature at the end of letters. It was used in order to signify that "God willing" this letter will get to you safely, "God willing" the contents of this letter come true. As an abbreviation (simply "D.V.") it is often found in personal letters (in English) of the early 1900s, employed to generally and piously qualify a given statement about a future planned action, that it will be carried out, so long as God wills (see James 4:13-15, which encourages this way of speaking). The motto of Southern Illinois University-Carbondale.
A much more detailed analysis with detection of relationships or clauses can be found in our Sentence Analysis! Try it out!