What does mc:Ignorable="d" mean in WPF?
The mc:Ignorable namespace provides xaml definitions that are "ignored" by the xaml processor.
This allows you to specify information used by the designer at design time which is ignored at runtime. In your case, you can specify DesignHeight and DesignWidth, which are not "real" properties on a Window, but work in the designer for providing a default design time experience.
mc:Ignorable="d" sets d: prefix as a mark for attributes used in design. Read more on MSDN: mc:Ignorable Attribute
d:DesignHeight="500" and d:DesignWidth="300" use that d: prefix, what makes them available only during design time - they are ignored after standard program compilation.