This is the 12th most frequent Italian word.
dei
"Dei" is the Italian word for "of the" (masculine plural).
In this sentence, 'dei' functions as a partitive article, equivalent to 'some' in English, indicating an unspecified quantity of books.
Ho comprato dei libri al mercatino.
I bought some books at the market.
Here, 'dei' acts as a contraction of 'di' + 'i', meaning 'of the', used to show possession or association of the colors with the flowers.
I colori dei fiori sono vibranti.
The colors of the flowers are vibrant.
In this instance, 'dei' is used again as a partitive article to mean 'a few', specifying an indeterminate number of days.