The phrase "few millimeters" means a very tiny measurement or distance. It is shorter than a centimeter and represents just a small amount, like the thickness of a coin or the width of a paperclip. Full definition
When they examined the xylem under a fluorescent microscope, they saw that bacteria had accumulated around pit membranes in the first few millimeters of the wood. (sciencedaily.com)
I am quite certain that some 98 % or 99 % of the energy in the electromagnetic energy that penetrates further than the first few millimeters of the ocean's surface is more or less instantly converted into heat. (wattsupwiththat.com)
I'm ready to sacrifice few millimeters to obtain excellent photos (from a smartphone point of view, clearly). (dxomark.com)