This is the 9737th most frequent Polish word.
zatrzymali
"Zatrzymali" means "they stopped" or "they detained" in English.
In this sentence, 'zatrzymali' is used to describe the action of stopping someone, in this case, by authority.
Policjanci zatrzymali kierowcę za przekroczenie prędkości.
The police officers stopped the driver for exceeding the speed limit.
Here, 'zatrzymali się' indicates a pause or stop made by the subjects themselves, in a reflexive form.
Zatrzymali się przy sklepie, aby kupić bułki.
They stopped at the store to buy rolls.
In this example, 'zatrzymali' means they ceased or halted an ongoing process, implying intervention to stop progress.