Indefinite articles are words used to refer to non-specific items or people. In English, the two indefinite articles are a and an. The choice between them depends on the sound that follows: use a before consonant sounds (e.g., a cat) and an before vowel sounds (e.g., an apple). These articles help indicate that the noun they modify is not a particular one. For example, saying "I saw a dog" means any dog, not a specific one. Indefinite articles are essential for generalizing and introducing new information in conversation and writing.