This is one of a multitude of cheapie shorts produced by Warners and other studios in the 1930s and 40s, featuring simple bandstand shots of 15 to 20 piece "swing" bands -- now called "big bands" -- playing 3 to 5 of their standard repertoire numbers. The productions were generally unimaginative, and might now be considered boring if you're unfamiliar with or dislike the music and bands of that era.
As a second-rate, classically oriented high school, college, and military band clarinettist and saxophonist in the 1938-46 era, I disdained the swing band music, though I did play some of it. I'm now a fan of swing and jazz, perhaps from the contrast with all the popular music trends that have followed. In particular, I hated Artie Shaw, no doubt from having seen this or other appearances in shorts and feature movies. Now I recognize that he was a fine performer, in spite of lifting his fingers ridiculously high and having his ligature upside down -- facts that I would not have learned from hearing his records on juke boxes. (Ligature: the clamping strap device that holds the reed on a clarinet or sax.)
This is an interesting and entertaining little movie. I love it -- now.