CSR is way you can modify the List View for the List. For example if you want any columns to show different thatn OOB SharePoint Column, you can use the CSR to achieve the custom column rendering.

I liked following articles for the CSR.

How to: Customize a list view in apps for SharePoint using client-side rendering

http://msdn.microsoft.com/en-us/library/office/jj220045(v=office.15).aspx

http://pratapreddypilaka.blogspot.com/2013/01/jslink-custom-js-rendering-in.html

Client Side Rendering using JSLink – Post 03 – Properties to override

http://jsuhail.blogspot.com/2014/09/client-side-rendering-using-jslink-post_30.html

http://sharepointandetc.blogspot.com/

Samples:

https://code.msdn.microsoft.com/office/Client-side-rendering-JS-2ed3538a