What is the use of ConvertBack method in IValueConverter interface?

ConvertBack is when you have mode=TwoWay on your binding.

It converts the user input back into the datatype that you're binding to and gets invoked when the TextBox (say) loses focus.

For example (if this wasn't built in), if you have a number that represents a price, the Convert method would take the value and format it into a string with the correct currency symbol, decimal separator etc.

If the user types in a new value - including possible currency symbol, thousands separator etc. you'd use the ConvertBack method to parse the string and extract the numeric value.

Another example could be credit card number input. The user could enter the number as a single string of digits or groups of digits separated by spaces or dashes. The ConvertBack method would take all these possible inputs and convert them to the single format you require.

IMO, the ConvertBack method is used to convert your visual representation of the data to the specific DataType.

For example: you use a Converter to convert a boolean true to the string "TrueBoolean". This text will be displayed in your TextBox. When you change the value of the TextBox, the ConvertBack method will be called as soon as the binding fires again (default OnFocusLost). Now your ConvertBack method will try to convert the new value to the datatype you want it to be. So you will have to implement logic to convert "FalseBoolean" to false.

public class Converter : IValueConverter
{
    object IValueConverter.Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (bool) value ? "TrueBoolean" : "FalseBoolean";
    }

    object IValueConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var s = (string) value;
        if (s.Equals("TrueBoolean",StringComparison.CurrentCultureIgnoreCase))
            return true;
        if (s.Equals("FalseBoolean", StringComparison.CurrentCultureIgnoreCase))
            return false;
        throw new Exception(string.Format("Cannot convert, unknown value {0}", value));
    }
}

This technique is used a lot in DataGrids if I'm not mistaken.

Hope this is a bit clear...

UPDATE
About you question in the comment:
To overwrite the default OnFocusLost binding behavior you have to change your binding like this:

<TextBox Text="{Binding MyText, UpdateSourceTrigger=PropertyChanged}"/>
<!--syntax might differ, can't access VS at the moment.-->