This is the 524th most frequent Romanian word.
fii
"Fii" in Romanian means "be" (as a subjunctive or imperative form of the verb "a fi" - to be) or "sons" (plural of "fiu" - son), depending on the context.
Here, 'fii' is the second person singular imperative form of the verb 'a fi' (to be), used to command or request someone to be in a certain state or manner.
Fii atent la explicațiile mele.
Be attentive to my explanations.
'Fii' is used in the subjunctive mood, second person singular, expressing a suggestion or necessity of being punctual.
Este important să fii punctual la întâlniri.
It is important for you to be punctual at meetings.
In this case, 'fii' is again the subjunctive mood, second person singular, expressing a state that is desired to be true.