If a place or a thing is jam-packed, it is so full of people or things that there is almost no room left — as if everything has been jammed in and then packed down. A jam-packed bus, a jam-packed weekend, a jam-packed suitcase. It is informal and lively; you would say it to a friend, not in a stiff report, where full or crowded would do. Packed already means “full”; jam-packed is packed even more tightly. The jam here is the verb jam, “to push things in hard,” not fruit jam.