This is the 16156th most frequent Polish word.
ukradli
"Ukradli" means "they stole" in English.
Here 'ukradli' is used in a narrative sense to indicate an action of theft performed by an unspecified group.
Ukradli samochód z parkingu.
They stole a car from the parking lot.
In this interrogative context, 'ukradli' refers to the act of stealing by the perpetrators.
Policja dowiedziała się, kto ukradli laptop.
The police found out who stole the laptop.
The word 'ukradli' here is used in a hypothetical situation suggesting guilt of theft.