Yes, it bridges the gap between the first and second movies.
Following directly after the events of How To Train Your Dragon but
before How to Train Your Dragon 2, Riders of Berk follows Hiccup as he
tries to keep balance within the new cohabitation of Dragons and
Vikings. Alongside keeping up with Berk's newest installment — A
Dragon Training Academy — Hiccup, Toothless, and the rest of the
Viking Teens are put to the test when they are faced with new worlds
harsher than Berk, new dragons that can't all be trained, and new
enemies who are looking for every reason to destroy the harmony
between Vikings and Dragons all together.
As for whether you can watch the movie without having seen the TV show or not, I'd wager to say you could. There may be some references you won't get, but I can't imagine the story writers for the second movie would make it completely impossible to enjoy without having watch 40 episodes of a tie-in TV show.