Seville Oranges are too bitter to be eaten out of hand or raw.
Bitter oranges such as Seville are valued for their intense orange flavour which comes through when you balance the bitterness with added sweetness, or extract or distill the taste.
Despite the name, Sevilles now are grown in many places other than Seville, such as Sicily and Malaga. They are grown almost exclusively for the marmalade industry, but you can still find them here and there in markets and also use them in sauces, ice creams and fruit desserts. Sauce Bigarade, for instance, the classic orange sauce served with duck, is made with Seville Oranges.
Sevilles have a very short season, starting to arrive in stores just after Christmas, being abundant and cheap by the end of January, and non-existent again a bit into February. Plan your marmalade making accordingly.
Another important bitter orange is Bergamot Orange, used in flavouring Earl Grey Tea.
Sevilles freeze well for cooking use afterward (but that’s okay, as that’s all you would ever do with Sevilles.)