"du" is the contraction of "de-le" = basically "of the" (+ masculine noun)
It is used in 2 cases:
PARTITIVE - to express "a piece of, a part of", ie when you don't use (eat/drink) the whole object. Ex: je veux du vin = I want (some) wine - je prends de l'argent = I take (some) money.
COMPLEMENT OF A NOUN - to mean "of the" including the translation of English possessive cases. Ex: le chien du garçon = the boy's dog / the dog of the boy. Ex: le bas de la page = the bottom of the page