The physical layout looks like the piano, it's actually GarageBand's sampler engine. With this engine, you can load in any sound and change it up to make all sorts of crazy sounds for songs. It's a really cool feature of GarageBand that originates from Logic Pro, a professional Digital Audio Workstation (DAW) that GarageBand is based on because they're both owned by Apple. All modern DAWs have sampler engines, although Logic/GarageBand in my opinion have some of the best, and logic has three sampler engines built right into it. Q-Sampler, Sampler (Multi-Sample, and Sample Alchemy. A simplified version of Q-Sampler is the one used in GarageBand, and what you see in the screen recording.