What does a blue and red arrow mean on music charts?

On music charts on Billboard Hot 100 what does a blue arrow pointing
up mean? Also what does a red arrow pointing down mean? And sometimes
it will be pointing up and down but one color, what does that mean?
What I mean is in this video:

http://www.youtube.com/watch?v=hFIwEUIR8L8feature=player_profilepage