This is the 766th most frequent Croatian word.
pišu
"Pišu" means "they write" in English.
In this sentence, pišu describes the presence of writing by unidentified agents on walls.
Na zidovima pišu različite poruke.
On the walls, various messages are written.
Here, pišu describes a repeated action by unnamed individuals in the present tense.
Pišu mi iz inozemstva svake godine.
They write to me from abroad every year.
Pišu is used to describe the action in the present tense performed by the children.