<DockPanel>
   <TextBlock FontWeight="Bold" Text="{Binding Path=Name}" /> //+ some text
</DockPanel>

<Grid.ColumnDefinitions>
  <ColumnDefinition Width="10*" />
</Grid.ColumnDefinitions>

<DataGrid Grid.Column="0" RowHeaderWidth="0" CanUserAddRows="False" AutoGenerateColumns="False"  x:Name="datagrid1" Margin="10,150,8,50" Background="Transparent" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" ItemsSource="{Binding}">

  <DataGrid.Resources>
    <Style TargetType="{x:Type DataGridColumnHeader}">
      <Setter Property="Background" Value="Black"/>
      <Setter Property="Opacity" Value="0.5"/>
      <Setter Property="Foreground" Value="White"/>
      <Setter Property="HorizontalContentAlignment" Value="Center" />
      <Setter Property="FontSize" Value="15"/>
      <Setter Property="FontFamily" Value="Arial"/>
      <Setter Property="Height" Value="50"/>
    </Style>
  </DataGrid.Resources>

  <DataGrid.Columns>
    <DataGridTextColumn Binding="{Binding ProductName}"      Header="Title"      MinWidth="50"   FontSize="16"  FontFamily="Verdana" />
    <DataGridTextColumn Binding="{Binding Quantity}"       Header="Quantity"    MinWidth="30"   FontSize="16"  FontFamily="Verdana" />
    <DataGridTextColumn Binding="{Binding NumberOfOrder}"    Header="Order number"  MinWidth="30"   FontSize="16"  FontFamily="Verdana" />
    <DataGridTextColumn Binding="{Binding User}"          Header="User"         Width="*"     FontSize="16"   FontFamily="Verdana" />
  </DataGrid.Columns>

  <DataGrid.GroupStyle>
    <!-- Style for groups at top level. -->
    <GroupStyle>
      <GroupStyle.ContainerStyle>
        <Style TargetType="{x:Type GroupItem}">
          <Setter Property="Template">
            <Setter.Value>
              <ControlTemplate TargetType="{x:Type GroupItem}">
                <Expander IsExpanded="True">
                  <Expander.Header>
                    <DockPanel>
                      <TextBlock FontWeight="Bold" Text="{Binding Path=Name}" /> //I am wondering what does this line mean?
                    </DockPanel>
                  </Expander.Header>
                  <Expander.Content>
                    <ItemsPresenter />
                  </Expander.Content>
                </Expander>
              </ControlTemplate>
            </Setter.Value>
          </Setter>
        </Style>
      </GroupStyle.ContainerStyle>
    </GroupStyle>
  </DataGrid.GroupStyle>
</DataGrid>