As someone who already knew the reason, this feels like a terrible explanation who will elucidate no one and will only make sense to those who already know what they’re talking about.
Each compartment contained a bunch of single metal characters (like the ones in the image at the top of the article) which were laid out by hand to form words and sentences and pages, rolled over with with ink, then a sheet of paper laid on top, then pressed to transfer the ink from the characters to the paper.
To try to be a bit more helpful, here’s a picture of the cases: https://i.imgur.com/9vxOtWB.jpeg
Each compartment contained a bunch of single metal characters (like the ones in the image at the top of the article) which were laid out by hand to form words and sentences and pages, rolled over with with ink, then a sheet of paper laid on top, then pressed to transfer the ink from the characters to the paper.