UPF stand for Ultraviolet Protection Factor. Clothing can protect you from the sun's ultraviolet rays. It all depends on the fiber content, weave, weight of fabric and color.
SPF stands for the Sun Protection Factor, most commonly associated with sunscreen. This measures the amount of time it takes for skin to burn.
UPF on the other hand stands for Ultraviolet Protection Factor, which is a measure of the amount of UV radiation that penetrates a fabric and reaches your skin. For example, a fabric rated with a UPF of 20 will allow only 1/20th of the sun's UV rays to pass through.
Marmot UPF fabrics come in UPF 20, 25, 30, 35, 45 & 50.
Wearing a UPF fabric would greatly reduce your exposure to UV rays, (where it covers your body).
Dark colors and tight weaves will have higher UPF ratings. As well, nylons and polyesters are better than plain cotton because they reflect radiation.