Determiners are words that come before nouns to specify or clarify something about the noun, such as its quantity, definiteness, or ownership. They function to narrow down the identity of a noun by introducing specificity, unlike descriptive adjectives which add detail. Common types of determiners include articles (the, a, an), demonstratives (this, that, these, those), possessives (my, your, their), and quantifiers (some, many, few).

Determiners are essential in a noun phrase as they provide crucial context such as how many of something there are, which particular one is being referred to, or who it belongs to. They appear immediately before the noun they modify and are sometimes called limiting adjectives because they limit or define the noun's reference.

In summary, determiners:

• Introduce or modify a noun
• Specify quantity, definiteness, or ownership
• Include types like articles, demonstratives, possessives, and quantifiers
• Are positioned before the noun in a sentence

This makes determiners a fundamental component of English grammar to clearly express meaning in sentences.