Pabst Blue Ribbon is a brand of beer currently manufactured by the MillerCoors alcohol brewing conglomerate. The style of Pabst Blue Ribbon is what is known as a lager, which is synonymous with "generic American beer". From my admittedly narrow knowledge and interest in beer, Pabst Blue Ribbon pretty much tastes and feels like any other mass-produced beer.
That being out of the way, Pabst Blue Ribbon's culture significance is way larger than its status as a totally middle-of-the-road corporate beer. For the past ten years, Pabst Blue Ribbon has become the beer of hipsters. At first, this was probably because hipsters were a lot of poor, scruffy kids who needed the cheapest beer they could find. It later became popular perhaps because its ironic association as a working class beer, and as a way for hipsters to work through whatever authenticity issues they always seemed so obsessed about. And after that, it probably became a metaironic comment as the hipsters of the late 00s made fun of the ironic hipsters of the early 00s. This is quite a pearl of reference to gather around the grain of "a pretty standard American beer", but such is the devious ways of hipster culture.