ECMASCRIPT is a method of (concept of) using JavaScript as Object Oriented designing. Knowing about this; help you to work on SharePoint Client Object Development, SharePoint Apps Development as well as any web development that involves JavaScript Objects)
There are few concepts you need to know. I will compare those using general OOP language (C#) and ECMASCRIPT. (C# vs. JavaScript)
Creating a Class
Creating a Object
Public and Private Properties
Method Definition (in best way)
(Note: Prototype is something like a template; we are assigning method save() to the template thus it will not recreate the everything in the browser memory every type we are creating a object. so assigning the method to the prototype is the best way to do it.)
Inheritance
I think, its good comparision. As a SharePoint developer, this mandatory to know these difference
Thanks SekThang!