Then you define a column in a XAML grid you can set the width to one of 3 possible values:
1. A fixed width,
2. Auto which means expand to the size of the content, or
3. * (star) take as much space as available
The * is prefixed by a number (default is 1 if no number is specified). The available space is divided among the starred columns in proportion to the prefix number.
If you have this definition
<Grid.ColumnDefinitions>
<ColumnDefinition Width="0.07*"/>
<ColumnDefinition Width="0.93*"/>
</Grid.ColumnDefinitions>
The first column will get 7% of the total space available and the second column would get 93%. On the other hand if you had this definition:
If you want a layout where the second column is double the width of the first and the third column is triple the width of the first you need this definition:
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="2*"/>
<ColumnDefinition Width="3*"/>
</Grid.ColumnDefinitions>
If the total width of the grid is 300 you get column widths 50, 100 and 150. If the total width of the grid is 600 you get column widths 100, 200 and 300. And so on.