We note that OSI-7 Layer Reference Model is the first topic to discuss in the course of learning how communication networks work.

There are many useful links for understanding the concept of OSI 7 layers. The following is a partial list for them:

[1]   https://www.softwaretestinghelp.com/osi-model-layers/ 

[2]   https://http://a-ztipsntricksparadigm.blogspot.com/2013/04/osi-model.html 

[3]   https://http://en.wikipedia.org/wiki/OSI_model

and much more on the web...