When you migrate someones email profile you move their PST file and setup their profile on the target machine. When you do this all the email and calendar items are moved but the email AutoComplete is not moved.
The email auto cache is where someone starts typing in an email and outlook auto fills the email or gives you a dropdown list. This cached list is stored in a NK2 file. This file needs to be migrated as well as the PST to maintain the drop down list.
If this list is not migrated the client can be left to use outlook normally and the list will populate over time and normal operation.