Mm means CSS millimeters, with 1mm = 1/10cm. Much like cm, mm are occasionally used when creating documents for later printing. For example, to specify the column and page dimen­sions as well as margin sizes.

Millimeters and other absolute units are not recommended for use on-screen because screen sizes vary a lot. For example, the same 60x60mm image will look different on a 24in desktop display and a 6in smartphone screen, not to mention that users look at them from different distances.