HAHAAHAHAH Eliot seems to answer all the chinese to english translations wrong! all these years eliot, and your Chinese has not improved! Terrible! :D The first explanation by Nobunyaga is pretty good. A literal translation of the 成语 is: 福如东海 May you have fortune as boundless as the eastern seas. 寿比南山 May you live as long as the Southern Mountains! i.e. Wishing you/May you be blessed with a long life and great prosperity.

福如东海 = lucky as immense as the East sea (idiom); a lucky sign 寿比南山 = Live as long as Mt Nan! (idiom, conventional greeting) / We wish you a Happy Birthday and many more of them.

It is a phrase used when you convey your birthday wishes to someone, usually an elderly person. I think there might not be an equivalence to this phrase in English but anything close to "Wishing you good fortune and longevity" is appropriate enough. For me, over here, a combination of "Happy Birthday to you!" and a big birthday hug usually does the magic. :D