In addition to Sravya's excellent explanation above where he displays a list belonging to a keyed property, you can also 'think' of Keyed Access as referencing a single record in a Database. Suppose you have an employee table with fifty records. The individual columns are FName, LName, ID, SSNum, and Salary and say you have a drop down list displaying all these employees ID Numbers. Say you want to find the details of the 28th employee.
1. You have a DataPage displaying all these fifty employee records.
2. The DataPage List is sourced by a Report definition which basically loads these fifty records.
3. In your Datapage you use Keyed page access and you pass the Key Property (whatever you are using in the dropdown list). Here you pass ID.
4. When you select the 28th employee, using keyed access, your report definition and hence the datapage will give the details of the 28th employee, their FName, LName, SS, ID and Salary. It retrieves the record belonging to that key. (i.e the ID property)
5. Suppose your List was displaying Employee SS Numbers. In your Data Page, you set the Keyed Page Access to the SS property. Your Report definition will now pull that record based on SS property.