The Mesh size is measured by how many threads there are per square inch. For Example, an 86 Mesh screen has 86 threads per square inch. The higher the Mesh count, the finer the holes are in the screen allowing finer detail but also reduces the amount of ink you can deposit. The size of the Mesh has a lot to do with how thick the ink you are using is. How detailed your image is also a consideration in the Mesh count selection.
If you have a design with extremely high detail, a lower Mesh screen won't hold the fine lines or dots in the image and they will simply not show up causing a lack of detail in the finished product. On the other hand, if you are trying to print a thicker ink (such as white) through too high of a screen Mesh, barely any ink will print through the Mesh as the holes are too small. You may notice that different companies have slightly different sizes available. If the Mesh count is fairly close, such as 156 vs 160, 196 vs 200, or 81 vs 86, the difference is so small that it doesn't matter because you won't see a significant difference in your final product.
