This is the 15730th most frequent Polish word.
szczęśliwymi
"Happy" (instrumental plural form).
Here, 'szczęśliwymi' functions as a predicate adjective describing the subject in the instrumental case.
Marzymy, by być szczęśliwymi przez całe życie.
We dream of being happy throughout our lives.
The word 'szczęśliwymi' is used as an adjective agreeing in case, number, and gender with the subject 'we'.
Zawsze jesteśmy szczęśliwymi, gdy spędzamy czas razem.
We are always happy when we spend time together.
In this context, 'szczęśliwymi' modifies 'ludźmi' as an adjective in the instrumental case.