Silverlight {Binding Key} - Please refresh my memory?

I'm sure this is simple, but what the hell am I missing here? Can someone tell me what the keyword "Key" is referencing in Text="{Binding Key}"? I know I learned about it previously, but I cannot seem to remember. I understand how databinding works, just stuck on this particular issue.

 <DataTemplate x:Key="groupItemHeader">
            <Border Background="{StaticResource PhoneAccentBrush}"
                    Width="432" Height="62"
                    Margin="6" Padding="12,0,0,6">
                <TextBlock Text="{Binding Key}" 
                           Foreground="#FFFFFF" FontSize="26.667"
                           HorizontalAlignment="Left"
                           VerticalAlignment="Bottom"
                       开发者_开发问答    FontFamily="{StaticResource PhoneFontFamilySemiBold}"/>
                <Border.Projection>
                    <PlaneProjection RotationX="-60"/>
                </Border.Projection>
            </Border>
        </DataTemplate>

{Binding Key} is shorthand for {Binding Path=Key}. This will return the value of the Key property on the current DataContext.