If somewhere is jam-packed, it is so full of people or things that there is no room for any more.

Why do we say jam packed?

In moderate to rapid informal speech, to avoid the “reset” necessary to pronounce the following p, the d following the collocation jammed packed would likely be elided, as in red and purple. The result would be jam packed, which may ultimately be the independent source for both Crocket and the American usage.

What does it mean when something is packed?

filled to capacity; full: They've had a packed theater for every performance. pressed together; dense; compressed: packed snow.