Null check condition in Where clause
If NULL value of @SubDesc1 means "return all records, do not apply filtering", then 

SELECT * 
FROM Table1   
WHERE Category = 'Category1'   
  AND [Description] ='ABC'   
  AND (    SubDescription1 = @SubDesc1
        OR @SubDesc1 IS NULL)


If NULL value of @SubDesc1 means "return only those records, where the field IS NULL too", then 

SELECT * 
FROM Table1   
WHERE Category = 'Category1'   
  AND [Description] ='ABC'   
  AND (    SubDescription1 = @SubDesc1 
        OR COALESCE(@SubDesc1, SubDescription1) IS NULL )