In an exchange transaction coin(s) of one kind were exchanged for coin(s) of a different kind; for instance a trade of bitcoin was swapped for litecoin. The price recorded is indicative (estimated) based on the market value of the 'from' coin had when they were exchanged.
On this screen it is possible to unclassify existing transactions (note this will unclassify both the to and from transactions); he main information of the transactions will be preserved and this will allow you to reclassify then as a different type of transactions without needing to re-enter them. i.e. Exchange -> Unclassified -> Trade.
You can auto use the auto-value feature for looking up historic values for the 'from' coin the amount of the 'to' coin is calculated from this, this can subsequently be overidden. When you change an auto valued coin with a past date the to amount will default to invalid (red) to accept this value select ok.
For more information on auto-value click here.