Answer:
"De" refers to "une" or a singular thing, while "des" refers to multiple
Explanation:
For example, "de fille" - the girl vs "des filles" - the girls, I believe
More on preposition, according to this source: https://www.lawlessfrench.com/grammar/de-preposition/
"Many verbs must be followed by 'de' in front of a noun or infinitive" & "some adjectives must be followed by 'de' in front of a noun or infinitive."