The breathability of a material is its capacity to wick away the water vapour generated by perspiration. During intense effort, it is, therefore, essential to wear very breathable clothing so you do not stay wet.
Breathability is measured by the RET, the resistance of a fabric to the evacuation of moisture. The lower this coefficient, the more breathable the fabric is:
RET ≤ 6: very breathable fabric
RET ≤ 12: breathable fabric
RET ≤ 20: fabric with low breathability