on (C# Reference)
The on contextual keyword is used in the join clause of a query expression to specify the join condition.
Example
The following example shows the use of on in a join clause.
var innerJoinQuery =
from category in categories
join prod in products on category.ID equals prod.CategoryID
select new { ProductName = prod.Name, Category = category.Name };
See Also
Concepts
LINQ Query Expressions (C# Programming Guide)