It's based on a documentary, The Man Who Would Be Polka King, and tells the true-life story of Polish immigrant Jan Lewan (pronounced Yahn La-vahn...or Jan, he doesn't care) who takes a local polka band and souvenir shop and decides to build it into an empire, with results that surprise those around him, including his family. It also features Jenny Slate as his wife, and Jason Schwartzman, his best friend in the band (who's a composite of a few people). In a Q&A after the film, the writers -- Maya Forbes (who also directed) and Wallace Wolodarsky -- said that they took big liberties with some story points, but that much of the film is spot-on. They quipped that the fact that the real Jan Lewan has been going around saying how accurate the film is and how he doesn't know how they could have gotten everything so right only speaks impressively to who he is, since they made up good portions of it.
Here's the trailer. It's respectable for presenting the movie well, though focuses more on the conflict when things eventually start to bubble up, and doesn't get across the carefully-building flow and effusive surface charm of the character as well as is ideal.
Here's a bonus video. While I enjoyed the film, my favorite parts are the polka numbers, since Jack Black throws himself into them with total gusto. This below isn't from the movie, but it's from his recent appearance on The Stephen Colbert Show where he performs one of the polkas -- and does so in character, so you'll get to see. In some ways, this is more fitting than the trailer. Just know that the band here is bigger, more professional and doesn't have the people in dancing chicken and dancing bear costumes.
And one final bonus. This is a 4-1/2 minute montage comparing scenes from the movie to sequences from the original documentary. (Jan Lewan at one point hired a cameraman to record much of his activities.)