Hello and welcome to our forums,
Global.asax is an optional file which is used to handling higher level application events such as Application_Start, Application_End, Session_Start, Session_End etc.
It is also popularly known as ASP.NET Application File. This file resides in the root directory of an ASP.NET-based application. At run time, Global.asax is parsed and compiled into a dynamically generated .NET Framework class derived from the HttpApplication base class.
The Global.asax file is optional. You create it only if you want to handle application or session events.
For more information about this, please have a look on the Microsoft "Global.asax Syntax" article.
Hope this helps.