Usually, we think of index seeks (a possible Query Execution Plan operation) as using the b-tree index to pick out a row and then quickly pass that row to the next operation. But the index seek has special powers: when the conditions are right, it can examine non-key columns and filter based on those values inside the index seek operation.
When this happens the index seek properties will have two predicates – a seek predicate (which details the b-tree portion of the seek), and a predicate (with details about the additional filter using non-key columns).
You can see this behavior with the following code:
DROP INDEX Production.WorkOrder.IX_WorkOrder_ProductID
CREATE INDEX IX_WorkOrder_ProductID
ON Production.WorkOrder (ProductID)
SELECT WorkOrderID, StartDate
WHERE ProductID = 75
AND StartDate = '2002-01-04'