A micron is a unit of length equal to one millionth of a metre, in other words one thousandth of a millimetre. Its scientific name is the micrometre. In the bag and packaging world, film thickness is almost always discussed in this unit; how many microns a bag is gives a rough idea of how much material went into it and what strength can be expected.
Thickness alone does not determine quality. Two films of the same micron value can behave very differently depending on their raw material and production conditions. For this reason, thickness is assessed together with the choice of raw material and the thickness tolerance. During production, thickness is monitored on the line; in quality control, it is measured on samples.