Dapo,
This is quite a vague question. An answer could take hundreds of pages and still not be complete.
I will assume that you diligently investigated all the SNP related topics located in help.sap.com (where one can find a lifetime's worth of information about SAP software and business solutions) in an effort to discover this answer, before posting this question.
So, first, tell me exactly which of the SNP Planning topics that you read through in help.sap.com that you found confusing? I would want to see the help.sap.com citation you read, and a detailed question about the cited topic.
Best Regards,
DB49