In English grammar, a determiner is a word or a group of words that specifies, identifies, or quantifies the noun or noun phrase that follows it. Basically, determiners come at the start of a noun phrase and tell more about what comes after it (or them, in the case of a phrase).
Determiners include articles (a, an, the), cardinal numbers (one, two, three…) and ordinal numbers (first, second, third…), demonstratives (this, that, these, those), partitives (some of, piece of, and others), quantifiers (most, all, and others), difference words (other, another), and possessive determiners (my, your, his, her, its, our, their).