"Media naranja" is the way you call your wife, or your husband, if you are a woman.
Something in English like "your better half"??
It is very common in Spain... I am not that sure about the rest of Spanish-speaking countries.
"Un amor de verano" is just like a summer affair, you forget by the end of September. "Tu media naranja" is kind of "your true love"... or something to that effect.
Like the song I posted the other day, by Joaquín Sabina, he said something on the order of "It took me 19 days to forget you... and five hundred and something nights too..." (I don't have the song in front of me... but something like that.)
