Prime number generator in C++ ( alex allain jumping into c++)

i just started learning C++ , I am an art major (i know) and i started learning c++ from Alex Allain's jumping into c++ and in that book i have understood most of the stuff , I am currently stuck on prime number generator and i am having difficulty in the grasping the algothrimhere is an image of the code .I am not able to understand it after the first for loop

I humbly request anyone to help me understand this code step by step. thank you so much