As you know every object retrieved using Linq to SQL is being tracked by the DataContext class for any changes that happen in the life time of the object in the application. However, this takes more processing and off course performance will be affected.
Suppose one time you want to load some records from the database and bind them to a data control without the need to do any changes, instead we only want to load the records and show them on a a web page or windows form.
Here we need to tell the DataContext class not to track changes on the following query that is to be excecuted:
// Disable tracking
db.ObjectTracking = false;
// Prepare a query expression
var q = db.Customers.Where( c => c.City = "London");
// Execute the query
foreach(Customer c in q)
As you can see this can save some performance in case you are loading lots of records!
Hope this helps,