What is a determiner?

A determiner is a grammatical word that introduces a noun. For example, a, the, many, every etc. Determiners a/an are called indefinite articles, which means that there is no definite reference to what the noun is referring to. For example, if I introduced a piece of text with 'an apple fell to the ground'. The reader is introduced to this noun 'apple' and therefore the writer, when referering to the same noun later on in the text, could now use the definite article 'the' i.e. 'the apple' and the reader will know which apple the writer is referring to without confusion.