The 3 articles in English are a, an and the. The learner has to decide noun-by-noun which one of the articles to use.
The word a (which becomes an when the next word begins with a vowel - a, e, i, o, u) is called the indefinite article because the noun it goes with is indefinite or general. The meaning of the article a is similar to the number one, but one is stronger and gives more emphasis. It is possible to say I have a book or I have one book, but the second sententence emphasizes that I do not have two or three or some other number of books.
The word the is known as the definite article and indicates a specific thing. The difference between the sentences I sat on a chair and I sat on the chair is that the second sentence refers to a particular, specific chair, not just any chair.
Many nouns, especially singular forms of countable nouns must have an article. In English, it is not possible to say I sat on chair without an article, but a demonstrative or possessive adjective can be used instead of an article as in the sentences I sat on that chair and I sat on his chair.