An is an indefinite article. It is the form used before a vowel or, in some cases, ‘h-‘. You may also want to see H with indefinite article for uses of an with certain words.
- You may find an used up to the nineteenth century, and beyond in some dialects, as a conjunction to mean 'if', frequently in such phrases as "an't please you" (~if it pleases you). This is now obsolete, though occasionally revived for literary purposes.