Supergirl is the third show in what is now known as the Arrowverse along with Arrow and the Flash. Ironically the Arrowverse is turning out to be more of a thing than DC's desperately formulated DC Extended Universe (their attempt to ape the MCU) so perhaps they should focus more attention on their television properties.
Supergirl is actually Kara Zor-El, cousin to Superman and sent to earth at age 13 to keep an eye on him. Her rockest was knocked off course and forced into the Phantom Zone (where one assumed she spent most of her time partying with General Zod et al) for 24 years.
Apparently all the criminals Supergirl's mother prosecuted are now hiding out on Earth. She also meets the Martian Manhunter (J'onn J'onzz) so it looks like DC is pulling out all the stops.
The show is one of the good ones in the DC universe and if you are a fan you will enjoy these great t shirts in mens, womens, and kids with long slleves, tank tops, hoodies, and basic shirts all the way up to size 6 x-large.