Initial Values are configured to (1) show cells, and (2) open cells for input. One common use for this is to e.g. show all periods even if they do not all have values from the valuetypes in the dataset.
Locked Values override Initial Values meaning that you can create an app where e.g. all periods for the current year are shown as Initial Values, but the months with actuals are locked.
IV sets
Initial Values are configured through sets. The example below is a simple one with one set (the most common scenario). This example shows how to create Initial Values (or editable 0s in the grid) for Periods tagged with Show = 1 (first image) and Product tagged with Active = 1 (second image). "Tagged" in this context refers to the values of the Properties on the mentioned Dimensions.