This is the 147th most frequent Polish word.
dużo
"Dużo" means "a lot" or "much" in English.
In this sentence, 'dużo' is used as a quantifier to indicate a large amount or number of something.
Mam dużo książek.
I have a lot of books.
Here, 'dużo' serves as an adverbial quantifier, describing the abundance of sunshine.
Dziś było dużo słońca.
Today there was a lot of sunshine.
In this context, 'dużo' quantifies an abstract noun, indicating a significant amount of time.