This is the 997th most frequent Polish word.
zaczęli
"Zaczęli" means "they started" in English.
The word 'zaczęli' indicates the beginning of an action, specifically starting a walk.
Zaczęli spacer po parku.
They started a walk in the park.
'Zaczęli' in this context shows the commencement of an activity, 'watching a movie'.
Po obiedzie zaczęli oglądać film.
After lunch, they started watching a movie.
Here, 'zaczęli' describes the start of an action, which is 'swimming'.