Exemple : Relative => You should be in the same folder. Folder 1 contains one page " page 1 " and an image "X" Folder 2 contains one page " page 2 "
You are in the page 2 and you write =>
<image source="X.png" />
You can't do this because you are not in the correct folder but you can write "/Folder1/X.png" or "../Folder1/X.png".
Absolute => Full path like http://Abundantcode.com/image.jpg
For windows phone developpement, you have uriking.relativeorabsolute property :D