c++ code for tic tac toe needs explaining

I have just started out as a C++ learner and have absolutely no prior experience. As part of an assignment, I must find a code that codes the game tic tac toe using C++ and explain what the code does. Below i have found the code so can someone chunk this up into portions (about ten) and just comment on what the code is doing and what it means, why it has been added etc. I would really really appreciate this, do remember I am an absolute beginner so i only need absolute beginner comments on what the code is doing every several lines of code. Thanks.