An indefinite adjective is a word that describes a noun or pronoun, indicating that the described word can have many possible subjects. For example, the word "some" in the sentence "I bought some peanuts" is used in such a way. In this example, it acts as an adjective, since it describes the noun "peanuts," and is indefinite as it indicates that the noun is imprecise. An indefinite adjective is typically used with a noun to make a statement less specific; this can be seen in the phrase "many people" in the sentence "Many people like this movie," rather than "My friend and I like this movie."
Much like other adjectives, an indefinite adjective is primarily intended as a word that describes a noun or pronoun. The way in which these words provide a description, however, is to reduce the level of specificity with regard to the word they modify. Common examples of an indefinite adjective include words such as "some," "many," "any," and "few." When each of these words is used with a noun or pronoun, they become part of a phrase, usually a noun phrase, and are considered with the word they describe when regarded grammatically.
An indefinite adjective is typically used before the word it modifies, such as the word "few" in the sentence, "I bought a few books." In this sentence, the subject is the word "I" and the predicate consists of a verb, "bought," and the direct object, "a few books." This same concept could be expressed as "I bought a book" or "I bought those books," in which case the direct object would be fairly specific. In the first of those alternatives, one specific book would be the object that was purchased, while in the second example certain indicated books would be the purchase.
The original example, however, includes an indefinite adjective to make the direct object less specific. "A few books," can refer to an indeterminate number, though it is generally considered a low value, and not to any ones in particular. The speaker is not indicating which books were purchased, in much the same way that the indefinite pronoun "someone" in "Someone bought a few books," does not reference a particular person.
An indefinite adjective can often be removed from a sentence in order to make the object that would be modified more clear. For example, rather than, "Let's watch some movie," the sentence "Let's watch a movie," indicates a somewhat more definite concept. A demonstrative adjective can be used in its place, such as "that" to indicate a very particular object, such as "Let's watch that movie."