Supergirl Season 1 aired this past TV season on CBS, and while it built up a fanbase, it did struggle with viewership, at least by CBS standards. That's why a few weeks after the Season 1 finale aired, it was announced that Supergirl would move to The CW in the fall, the home of fellow DC shows Arrow, The Flash and Legends of Tomorrow. However, not everyone who watches the other CW DC shows necessarily saw Supergirl the first time around. That's why later this summer, Supergirl Season 1 in its entirety will re-air on The CW as a way to catch up.
It was announced today that starting on August 1, The CW will air two Supergirl reruns each Monday, so that by the time Season 2 premieres on October 10, all 20 of the Season 1 episodes will have had their time to shine at the show's new home. Hopefully the network will also post those episodes on their website for a few weeks, in case certain folks weren't able to watch them live or record them. Regardless, this is a great opportunity for people who missed Season 1 the first time around to finally experience them, and for existing fans, they can enjoy Kara Zor-El's first adventures again.
Many fans were pleased when Supergirl was moved from CBS to The CW, and by airing those Season 1 episodes in the late summer and early fall, it allows the show to establish a presence before Season 2 kicks off. Obviously existing fans could have bought Season 1 on Blu-ray or presumably found it on Netflix at some point, but this move isn't for the people who have already watched the show. It's for the people who heard about Supergirl when it was originally airing, but didn't have a chance to delve in. Who knows, maybe the re-airing will also hook some people who are just switching channels on a Monday night and stumble upon the Girl of Steel's exploits in National City.
As for Supergirl Season 2, its plot is still being kept under wraps, but among the things fans can look Supergirl to are Superman being fully seen in the first two episodes (played by Tyler Hoechlin) and new characters like Lena Luthor and Snapper Carr being introduced. Although Supergirl will be involved in a massive crossover this winter with Arrow, The Flash and Legends of Tomorrow, it hasn't been officially confirmed yet whether the Supergirl universe will be merged with Earth-1 or will remain separate for a while.