The OK BOOMER is another prototype/development board for testing the LM4875 audio amplifier, part of TI's "Boomer" series of simple audio amplifiers. It's designed for inexpensive, low power audio applications that output to speaker and optionally a headphone jack.
In the application I was testing, audio came from a Raspberry Pi. The Raspberry Pi doesn't actually have an audio DAC on board. It instead uses PWM and a passive RC filtering network to create an analog output. The OK BOOMER has the same filtering so that it can be connected directly to a Raspberry Pi whose outputs have been reconfigured to the GPIO pins. This is especially useful for Pi Zeroes since they don't have any of this circuitry. It can just as well amplify any other audio signal, with some attenuation from the filtering.