Sometimes you want to bind to a property in the ViewModel, but you want that to be altered before it is set on the View Property. You can use a Converter to make a string all lowecase/uppercase with TextTransformationConverter or check if an enum equals a certain value with EnumToBoolConverter.
Some Converters have parameters so you can tailor the output to a specific Property.
You can write your own converters by inheriting from ValueConverterBase. You then need to implement abstract methods Convert and ConvertBack (ConvertBack is only used by TwoWayDataBindings)