What is spin spin coupling?
As it stands, your question is very general. It is unlikely that a single answer here will describe spin coupling and multiplicity in a simple and detailed manner any better than any publicly available reference source. There are hundreds available. I suggest you read one or more of the following extremely useful introductory NMR references, and ask more specific questions that you do not understand from them.


Reich (http://www.chem.wisc.edu/areas/reich/chem605/)
Hornak (http://www.cis.rit.edu/htbooks/nmr/) 
Libretexts (https://chem.libretexts.org/Core/Organic_Chemistry/Spectroscopy/Nuclear_Magnetic_Resonance_Spectroscopy
and
https://chem.libretexts.org/LibreTexts/Purdue/Purdue%3A_Chem_26200%3A_Organic_Chemistry_II_(Wenthold)/Chapter_12._Nuclear_Magnetic_Resonance)