It's called the AUDynamicProcessor, and it's very helpful when it comes to mastering your songs in GarageBand. In this video tutorial, you'll learn how to utilize and manipulate the sound dynamics of your songs in GarageBand using the AUDynamicProcessor filter, which effects the high and low ends of audio.
The AU Dynamic Processor is basically a compressor / limiter that affects the dynamic range of sound frequencies. This is great for drums or tweaking vocals, if you're looking for a different kind of resonant sound. Also in this video, learn how to set the threshold ratio, attack time, release time and headroom to optimize the sound of your music projects.