This is the 565th most frequent Italian word.
pensa
"Thinks" (third person singular of "pensare").
In this sentence, 'pensa' is the third-person singular form of the verb 'pensare' in the present indicative tense, meaning 'thinks'.
Maria pensa che tu sia molto bravo.
Maria thinks that you are very talented.
Here, 'penso' is the first-person singular form of 'pensare', and it means 'I think'.
Quando ci penso, mi viene nostalgia.
When I think about it, I feel nostalgic.
'Pensa' is used as an imperative form (second person) here, meaning it is a command or suggestion, translating to 'Think'.