Hi aman,
There is a mistake in your code. You missed to write the Path in Binding expression. See the below updated one. Notice the changes in bold text.
<StackPanel HorizontalAlignment="Center">
<Slider x:Name="slider1" Minimum="30" Maximum="300" Value="150" Width="300"/>
<Rectangle Fill="Black" Height="200" Width="{Binding Path=Value, Mode=OneWay, ElementName=slider1}"/>
<TextBlock FontSize="24" Text="{Binding Path=Value, Mode=OneWay, ElementName=slider1}"/>
<TextBox x:Name="tbage"/>
<HyperlinkButton Content="Enter Site" Visibility="Visible"/>
</StackPanel>
Let me know the results.