This is the 12175th most frequent Polish word.
martwiæ
"Martwić" means "to worry" or "to be concerned."
The verb "martwić" is used reflexively here as "martwić się", meaning "to worry" about something.
Nie musisz się martwić, wszystko będzie dobrze.
You don't have to worry, everything will be fine.
In this sentence, "martwić" is used transitively, meaning "to worry" or "to concern" someone.
Martwi mnie jego zachowanie ostatnio.
His behavior lately worries me.
Here, "martwią się" is the reflexive form used in plural to express that parents generally worry, showing concern for someone.