As you know, the original series of Seinfeld ran from 1989 to 1998, and was written by Jerry Seinfeld and Larry David, later of Curb Your Enthusiasm fame. The show distinguished itself by its postmodern absurdism and observational humour. The creators chose to close the show while it was still number one in the television ratings.
After Seinfeld, there was talk of a "Seinfeld curse" as every star's initial attempts at another sitcom failed.
Seinfeld went on to start The Marriage Ref, make Bee Movie and other less successful works. Jason Alexander and Michael Richards never got anything great together, and in fact Richards embarrassed himself with failed race gags at a stand-up performance.
Julia-Louis Dreyfuss managed to break the curse, though. She had initial difficulty, but later hit gold with The New Adventures Of Old Christine and later Veep, earning Emmys for both.
This Seinfeld reunion show happened in 2009, a decade after the final "official" episode of Seinfeld in 1999. It ran as a recurring plotline in series seven of Curb. As you can tell by the Madoff refs, this is topical, but the observational humour and relationships are as they were in the legendary Nineties sitcom.