UPF40 stands for "Ultraviolet Protection Factor" 40, which indicates the level of protection the fabric provides against harmful UV rays from the sun. A UPF 40 rating means the shirt blocks approximately 97.5% of the sun's ultraviolet radiation, allowing only 2.5% to penetrate through the fabric. This level of protection significantly reduces the risk of sunburn and long-term skin damage, making UPF 40-rated shirts an excellent choice for extended sun exposure during fishing and other outdoor activities.