There are two types of dynamic values: Computed values which compute a value based on other fields and filtered values which filter user input.
The first example filters away all non numbers which you enter:
In this example, we compute the sum of Field 1 and Filed 2 and display it in the Sum field:
And you can compute collection item specific data, as well:
Another special type of dynamic value is a field using the "precision" option, which is very useful for price values. Try entering some other value, for example "40.1234" or just "40".