what is session time out?

When a user wants to retain data in the server, for specific purpose this feature known as Session state. A session is defined as the period of time that a user interact with the web application. As the logically , session state is nothing more then a memory, which keeps the data set for read duration of user's session. 

example

Session("Name") = "Tin Gyi";


there are some state of sessions :-

Mode. 
Cookieless. 
Timeout. 
Sqlconnectionstring. 
Server. 
Port. 


if you wants to learn more about the session then please 
read this article carefully http://www.c-sharpcorner.com/UploadFile/5d065a/state-management-technique-in-dot-net/