They are namespace prefixes -
MSDN documentation[
^]
x: is likely to be
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
m: is likely to be
xmlns:m="clr-namespace:Microsoft.Maps.MapControl.WPF;assembly=Microsoft.Maps.MapControl.WPF">
Have a look at the top of your XAML and you will see the namespaces you are using listed in the Window tag.
In your example
x:Type
is used instead of
Type
to distinguish it from the one in
xmlns:system="clr-namespace:System;assembly=mscorlib"