A is used before consonants, an before a vowel sound. A comes before words that begin with a u, but are pronounced as though they began with a y: a union; a useful gadget. An comes before a silent h: an heir; an honour. Some people still use an before h in words from French, where the h was silent: an hotel. This is rather old-fashioned. There is no reason to use an before an h which is sounded.
The stars represent the states of Pohnpei, Kosrae, Yap, and Chuuk. The blue field is said to represent the Pacific Ocean. Effective date: 30 November 1978.