The general meaning is that it's short for "daran", which in turn is the replacement for "an + X" (like auf X -> darauf, über X -> darüber, etc.). There are plenty of specific meanings, though, for example "du bist dran" = it's your turn (from "an der Reihe sein"); "an der Sache ist nichts dran" = this affair isn't genuine/important; and many more. More meanings in your dictionary: http://de.pons.com/%C3%BCbersetzung?q=dran&l=deen&in=ac_de&lf=de