This is the 5210th most frequent Catalan word.
belles
"Beautiful" (feminine plural).
In this sentence, 'belles' is used as an adjective to describe the plural feminine noun 'flors' (flowers), meaning 'beautiful'.
Les flors són belles.
The flowers are beautiful.
Here, 'belles' operates in the superlative form to indicate that a group of feminine subjects (elles) possess the highest degree of beauty.
Elles són les més belles del grup.
They are the most beautiful in the group.
In this context, 'belles' is part of the noun phrase 'belles arts', referring to the fine arts, so it modifies 'arts' (a feminine plural noun) according to its specific idiomatic usage.