Orange Blossom Water is unusual in most American cooking, but that means when it makes an appearance it's really special.
Orange Blossom Water is made from water infused with oils from the blossoms of bitter orange. It is similar to but not the same flavor as orange zest. It is common in middle eastern cooking.
Because of its (to Americans) exotic flavor, it makes an interesting ingredient for cocktails like the classic Ramos Gin Fizz. I also use it in the Mexican Pan de Muerto (traditional bread for the Day of the Dead festival) and in Armenian Kadaif (a dessert of shredded fillo and custard).
I always use Cortas brand that I buy at Bahnan's Market in Worcester. Buy a small bottle; you use it infrequently and in small quantities, so a 10-ounce bottle lasts a long time.