Der, die and das are definite articles like "the" in Englisch: https://deutsch.lingolia.com/en/grammar/nouns-and-articles. German nouns have three genders: masculine, feminine and neuter. You have to memorise the gender for every word as well as the declensions but there are cues that help you to know the gender in many cases: https://deutsch.lingolia.com/en/grammar/nouns-and-articles/gender.

Der Junge - Die Jungen 
Das Mädchen  - Die Mädchen 

Das Kind - Die Kinder
Der Mann - Die Männer 
Die Frau - Die Frauen 

Thanks very much. What is the method to learn das, die, etc. Das and  die like "The" in English?