Whenever you make a change, ModelRight uses an internal "transaction" to contain the full impact of your change. When the transaction completes, it evaluates all of these changes and inserts an entry to this tab. Any entry can be expanded so that to see the full impact of your changes.
You can specify the number of transactions to view in the dialog in the Tools->Options dialog.
If you right click on any of the transactions, a context menu will be displayed. If you select the Undo option, ModelRight will roll-back all the transactions up to and including the one you selected.