Mesh size refers to the numbers of openings per inch on the insect screen, warp and weft
For example, 18x16 mesh means 18 holes per inch in warp and 16 holes per inch in weft.
More holes per inch means more dense the insect screen is.
So 18x16 mesh insect screen is more dense than 18x14 one, 20x20 one is more dense than 18x16 one.
And more dense, the price will be higher.