An indefinite article is a tiny but important word that comes before a noun when you're talking about something general rather than specific. In English, the indefinite articles are a and an.

When you say “I saw a dog in the park,” you're not talking about any particular dog that your listener already knows about. You're just mentioning some dog, any dog. Compare that to “I saw the dog in the park,” which suggests a specific dog you've both been discussing.

The difference between a and an depends on the sound that follows. Use an before vowel sounds: an apple, an hour (even though hour starts with h, it sounds like it starts with a vowel). Use a before consonant sounds: a banana, a university (even though university starts with u, it sounds like “yoo-niversity”).

These little words are called “indefinite” because they don't define or specify which particular thing you mean. They're called “articles” because they're a special category of words that always attach themselves to nouns. Many languages have indefinite articles, though they work differently. Some languages, like Russian and Japanese, don't use them at all.