What is the purpose of filestream, memorystream and save connection string into webconfig

Question

hi...
string conn = ConfigurationManager.ConnectionStrings["Connection"].ConnectionString;
what pupose we are using this statment ?, what are the perferred necessary steps to use this statment ?.
exaplain to me with simple one example..
 
and also purpose of,
1)File stream
2)Memory stream
 with example.....