The Jamstik App has a palate of sounds that you can play right away to get you started. When you first download the app, four sounds will be automatically available—including Bright Grand Piano, Classical Acoustic, and Steel Strings.
The menu also includes an "Off" selection in case you want to run the Jamstik app in the background silently while you play other MIDI apps on your device.
This allows you to use the app's control features without the apps sound engine interfering with your work in another app, such as GarageBand.
If you'd like to download additional free or paid sounds for the app, check out this article.