Il partitivo

L’articolo partitivo (partitive article) is expressed by the preposition di combined with the definite article. It is used to indicate undefined or approximate quantities and may appear before singular nouns (del latte, dello zucchero, della farina) as well as before plural nouns to indicate some part of a category or an unspecified number of things/persons (dei libri, delle ragazze, degli studenti), thus working as plural of indefinite articles.

For example:
Maria beve del caffè - Maria is drinking some coffee
Vuoi dello zucchero? - Do you want some sugar?
Ho delle cravatte blu - I have a few blue ties
Esco con degli amici - I go out with some friends