Relative Layout and xaml :: Why am I having such a hard time finding examples and explanations?

I've found a multitude of Classic Xamarin examples and a few xaml examples but what I'm not finding (and I could be searching wrong) are examples that explain the ins and outs of X and Y Constraints.

I'm very new to this and to C# a little, so it's possible I'm looking for this info in the wrong spots.

All I want to do is take an image, overlay a boxview with a color opacity and drop a label centered over that.

I understand conceptually how to do it, but I'm struggling to understand the details of factor, constant, etc...

Any help with this would be amazing.

<RelativeLayout>
    <Image 
        Source="compliance.jpg"
        x:Name="complianceBG"
        />
    <BoxView 
        Color="#80000000"
    />
    <Label
        WidthRequest="667"
        Text="{Binding Title}"
        FontAttributes="Bold"
        FontSize="18"
        TextColor="White"
    />  
</RelativeLayout>

Thanks,
Steve