child bptm, shared memory and parent bptm

Hello,
I need to know about shared memory. What is it exactly.
During backup bpbkar sends data strems to child bptm. Then child bptm store that strems in shared memory and from shared memory parent bptm took the data and write to media.

So will you please help me to understand about shared memory. What is it...

Or it is just disk storage from our hard drive. Which get in use as shared memory for temporary basis to store the data strems.