Document.ready refers to when the DOM object model is ready to be manipulated by JavaScript, like linking an event to an action, like the Keydown event and you have a JavaScript faction you want done based on what key was pressed, as an example.
https://en.wikipedia.org/wiki/Document_Object_Model
Window.Load is like the Form_Load() in a Windows form.
https://coderwall.com/p/_jothq/jquery-when-to-use-document-ready-and-when-window-load
Document.Ready fires first.
However, the question you ask are off-topic concerning the C# forum. Web questions and issues can be discussed at the ASP.NET forums.
https://forums.asp.net/