Sound on Sound recording lets you record without erasing the existing part (ie. the recording is added to the existing part). This can be used to add some notes to an existing MIDI part. It also works with audio. The existing part is audible, except when using an External MIDI Instrument.
Sound on Sound recording can be turned on/off using the SoS button at the top of the window.
After stopping the transport the recording appears in the track's editor as a tweakable edit, so you can resize the selected part, undo the recording or use the editor's Edit button to change the recording's volume etc. Note that only the newly recorded part is affected by these actions, the original part remains untouched.