periodic boundary conditions

I’m a beginner in Netgen. I’m not able to understand boundary conditions and periodic boundary conditions. When we create a structure, what does it mean when we give " -bc=1" and in one tutorial I saw that in the end they have given like
“identify periodic p1 p2;
identify periodic p3 p4;
identify periodic p5 p6;”
So, if we want to make periodic boundary conditions for 2 identical structures what should we do???

Thanks in advance