Answer
A ResourceDictionary stores reusable resources (colors, styles, templates, converters) keyed by x:Key. Define it at app, page, or control scope; reference resources with StaticResource (resolved once) or DynamicResource (tracks changes). App-level resources in App.xaml are available everywhere.
<Application.Resources>
<ResourceDictionary>
<Color x:Key="Primary">#512BD4</Color>
</ResourceDictionary>
</Application.Resources>
<Button BackgroundColor="{StaticResource Primary}" />Real-world example Brand colors defined once in App.xaml are reused across every page via StaticResource.