Mass includes a simple internal synth to use as a reference when you are without other gear to jam with. It can be enabled and disabled in the General Settings block, and is enabled by default when you install the app. This setting is persistent.
Note: the internal synth is not built into the AUv3 version of Mass.
As of version 1.0.0 of Mass, the synth has no exposed parameters. This may or may not change in future updates.