“Jam” (mermelada) you may know, and you may know the word “packed” (infinitive = pack).
“Jam-packed” (abarrotado, atestado) can refer to a room full of people so there is little space.
A room can also be jam-packed (repleto) with things, that is, completely full of things. Another way to say the same would be “jam-full”. A cupboard can be jammed-full of things.