The <LineBreak></LineBreak> tags allow you to insert line breaks at desired location of content inside a TextBlock.
Consider the following example,
<UserControl x:Class="Demo.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
<Grid x:Name="LayoutRoot" >
<TextBlock>The function of the DescriptionViewer
<LineBreak></LineBreak>control in
Silverlight is to display some
<LineBreak></LineBreak>information as a description and
<LineBreak></LineBreak>track error state for an associated
<LineBreak></LineBreak>control.</TextBlock></Grid>
</UserControl>
To see how the LineBreak tags work, also test the above code without the LineBreak tags. Then you will understand the difference.