Many of the people attending our jam have no idea what this means. There are some great explanations (although lengthy) on the web such as What is Old Time Music?, wiki/Old-time Music and Old Time Music.
The best explanation is shown in the video clip from a documentary called "Why Old Time". This film looks at the living heritage of music from the mountains of Appalachia that has spread all across the world.
Squirrel Heads and Gravy by Chris Germain.