My friend recently asked me this question and here was my answer:
Del.icio.us is an online bookmarking site. Similar to the way you can "bookmark" a webpage in your browser's Favorites, you can bookmark a webpage on your own del.icio.us page, allowing you to access your bookmarks from any computer.
I was wrong. What is delicious? Here you go:
Blood oranges. That is what is delicious. Shows you how my mind works these days.
(Thanks to Hannah for asking.)