UPF is similar to SPF (sun protection factor) used to rate sunscreens but UPF is the rating used to measure the amount of UV rays that pass through fabrics when exposed to UV Radiation or UVR.

To be called sun protective fabric it must achieve a minimum UV Protection Factor (UPF) rating of at least UPF15 after the equivalent of 2 years of normal wear and tear. 

Where you are protected by an IdleBird garment - you are UPF50+ protected

Source www.skincancer.org