The indefinite articles a and an are used to refer to a general or unspecified version of a noun (e.g., a house). Which indefinite article you use depends on the pronunciation of the word that follows it.
A is used for words that begin with a consonant sound (e.g., a bear).
An is used for words that begin with a vowel sound (e.g., an eagle).
Indefinite articles can only be used with singular countable nouns. Like definite articles, they are a type of determiner.