When a noun is used definitely, we know which person or thing is being talked talked about. In he following sentence, for example, we already know that there is a new Tarantino film.
Heb je de nieuwe film van Tarantino al gezien?
Have you seen the new Tarantino film yet?
Dutch has two words for the definite article (English the), namely de and het. De is used with common gender nouns (i.e. masculine and feminine nouns) and all plurals. Het is used with neuter gender nouns. We also talk about de-words and het-words.
There are some rules for recognizing which nouns are common gender and which are neuter, but it is easiest to check the gender of a word (e.g. in a dictionary) and to learn it for every individual noun. One rule is that all diminutives are neuter gender words (for example het stukje = the piece). Knowing the gender of a word is very important, because it has repercussions for the declension of attributive adjectives (i.e. adjectival endings).