A user defined value associated to an item.
A field can be set to any value you want.
The ItemField property allows you to add custom key/value pairs to any single item. This is a great way of adding metadata to an item for later use. For example, let's say your tree control contains human readable names of available application windows. You could then associate the internal form names using this property. The options are endless.
Note also that user defined fields can be used in any property that supports Conditions.