UV stands for Ultraviolet radiation, which has wavelengths shorter than visible light, thus making it invisible to the naked eye.
These wavelengths are classified into 3 groups:
UVA, UVB, UVC.
UVA is the longest, UVB middle, UVC shortest wavelength
UVC is absorbed by the ozone layer (it doesn't reach the earth!), so UVA and UVB are our main concern for skin cancer and other skin diseases.
A broad spectrum sunscreen is one that "provides proportional protection against UVA and UVB rays". Yes, that how a sunscreen can pass the FDA's Broad Spectrum Test.
In other words, broad spectrum means UVA and UVB protection. Simple as that.
I hope this helps!