Anything that's jam-packed is as crowded or full as it can be. If you hate crowds, you'll want to avoid the jam-packed shopping malls around Christmas time.
A subway car in Tokyo at rush hour is jam-packed, while a bus in a small village in Germany may have only a few passengers. And a dull day in traffic court will be sparsely attended, but during a dramatic celebrity trial the courthouse is bound to be jam-packed. This adjective is informal, dating from about the turn of the twentieth century. Earlier it was jam-full — both come from the "squeeze into a space" sense of jam.