Overdubbing is much like what you have done previously with the background music, but you can do this with any additional voice tracks or sounds that you want to add to your podcast. Simply, overdubbing is the idea of recording another audio track and placing it "over" the already-existing track. In Audacity, you can even record another audio track starting anywhere within the timeline. To start an overdubbing session:
Select where you want the overdub to start, in the current timeline.
Click on the Record button to start the overdub. Notice that visually you will see the overdub recording start where you specified, and you will simultaneously hear the audio of the initial audio track as you record the new overdub track. You can talk as you normally would, if you want to add in some additional "side-talk", or you can make some additional sounds (like clapping or walking) to accentuate your "story" in the podcast content track.