Script Runner allows you to write your code to text field a run it, without a need to make new c# script file.
It is very helpful when you need to instantly debug some value, or make some manually tedious task inside of an editor. For example, build your scene made of 100 spheres with different scripts attached or materials is a no longer slow manual task.
Editor Code Runner has 2 options to run your text written c# code. One is to run simple lines commands, like for example Debug.Log(). The advanced option is to write more complicated code which has more methods inside.