This is the 15750th most frequent Polish word.
szli
"Szli" means "they walked" or "they were walking" in English.
Here, 'szli' is used to ask about a past action of a group of people (masculine gender group) involving movement to the store.
Czy oni szli już do sklepu wczoraj?
Did they already go to the store yesterday?
In this case, 'szli' indicates an ongoing past movement of a group of people (masculine gender group) along the street.
Gdy szli ulicą, zauważyli coś ciekawego.
While they were walking down the street, they noticed something interesting.
The word 'szli' is used to describe a past continuous movement by a group of people (masculine gender group) towards the park.