An indefinite article is a limiting adjective that precedes an unspecified thing, person, or idea. In English, the only indefinite articles are a and an. The article always comes first in its noun phrase, sometimes preceding other adjectives in the phrase (for example, a precedes the adjective black in a black dog). A is used when the following word begins with a consonant sound (e.g., a dog, a historic event, a yeti), and an is the correct article when the following word begins with a vowel sound (e.g., an old man, an hour, an elf).
Contrast indefinite articles with definite articles, which precede understood or definite things. In English, the only definite article is the. When you say the black dog, for example, your listener should know which black dog you’re talking about. A black dog is an unspecified black dog. There are millions of them.