A millimeter is a unit used to measure very small distances, like the size of an ant or the width of a paperclip. It is abbreviated as mm and is equal to one thousandth of a meter. Full definition
That being said, they only add a couple of millimeters of thickness — something that many will welcome, given it makes the camera sit flush with the rest of the back, and not everyone prefers such slim devices anyway. (xda-developers.com)
But indeed, they managed to grow the crystals measuring a few millimeters in diameter (a small fraction of an inch). (sciencedaily.com)
The sides of the two screens match closely and the combined «bezel» separating the screens is just a couple of millimeters thick. (androidauthority.com)