It's the theme of the line of merchandise for this year. Last year was 'Crystal Empire', which tied in with season 3. Rainbow Power is suspected to somehow tie in with Season 4, but mostly ties in with the next Equestria Girls movie, Rainbow Rocks.
This pretty much means that the toy ponies will be decked out in rainbow-y tattoos (much like Stinky Pete à la Barbie at the end of Toy Story 2) for the rest of 2014.
