about radiomic features

Hello everyone, I have some questions about the definition of radiomic features on offcial website of pyradiomics. here is the sreenshots

I don’t understand what p(i) means. To my understanding, p(i) is group of value of intensity in a bin.but i am not sure. In addtion, I am not very clear about the difference of X(i) and p(i). Maybe I would like a exact example if possible or some detailed references. Thank you very much If someone can help me. looking forward to your reply.