This is the 177th most frequent Italian word.
grandi
"Grandi" means "big" or "large" in English, and can also mean "great" or "adult" depending on context.
In this sentence, 'grandi' functions as an adjective describing 'progetti' (projects), meaning 'big' or 'large'.
I progetti grandi richiedono tempo.
Big projects require time.
Here, 'grandi' is again an adjective, attributing the quality of being significant or important to 'opportunità' (opportunities).
Le opportunità grandi sono rare.
Big opportunities are rare.
'Grandi' describes 'bambini' (children), specifying those who are older or more grown-up.