The number of holes (not wires) per linear inch going across the width of the material x the number of holes per linear inch running down the length.

Let us introduce wire mesh size, 6 mesh indicates that in one inch there are 6 openings. You could easily pass a pencil through the openings.

22 mesh indicates that in one inch there are 22 openings. This mesh is used in a EMI & RFI shieldong screens.

100 mesh indicates that in one inch there are 100 openings and it is used in oil filters.