Low mesh counts use thicker threads with bigger openings, so they pass a heavy layer of ink. Printers choose them for puff, glitter, shimmer and bright whites on dark shirts. High mesh counts use finer threads with smaller openings that hold tiny dots and thin lines, which suits halftones, water-based inks and soft prints. Ink data sheets list the range each ink is designed for.
Mesh is one reason identical artwork can feel different coming from two printers: one prints through coarse mesh for punchy coverage, another through fine mesh for a lighter hand. Stencil thickness from the emulsion and the squeegee setup change the deposit as well.
You rarely pick the mesh yourself, but you can tell the printer what matters more to you: maximum brightness or a soft feel. In the US, mesh is usually quoted in threads per inch; European suppliers often use threads per centimeter. The mesh count guide goes deeper.