If you've never heard the term before, "catfishing" is when someone online uses false information, and maybe even a false identity, to try and secure some sort of romantic connection with someone else.
Basically, it's lying, and it's become so rampant in today's online world that there's even an entire television show dedicated to outing those people pretending to be someone they're not.
Usually in those cases, the person who is being catfished doesn't actually know the catfisher in real life. But when one girl found out she'd been deceived on Tinder, she was shocked to discover the catfisher was none other than her own brother.