De man leest een boek.
English: The man reads a book.
In Dutch, 'lidwoorden' are words used before nouns to indicate if you are talking about something specific or something in general. There are definite and indefinite articles.
Use 'de' or 'het' when you talk about a specific thing or person (the). Use 'een' when you talk about something non-specific (a or an). The choice between 'de' and 'het' depends on the gender of the noun.
De man leest een boek.
English: The man reads a book.
Het huis is groot.
English: The house is big.
Een hond loopt in de tuin.
English: A dog walks in the garden.
De appel is rood.
English: The apple is red.
Het kind speelt.
English: The child plays.