This is the 15404th most frequent Polish word.
spały
"Spały" means "slept" (past tense, plural) in English.
In this sentence, 'spały' indicates the past tense third-person plural feminine form of the verb 'spać', meaning that the girls were sleeping.
Dziewczyny spały długo po ciężkim dniu.
The girls slept for a long time after a hard day.
Here, 'spały' is used to describe the action of sleeping in the past for third-person plural feminine subjects, in this case 'my cats'.
Czy wiesz, gdzie spały moje koty wczoraj?
Do you know where my cats slept yesterday?
In this instance, 'spały' is referring to the past completed action of sleeping, done by third-person plural feminine subjects, here 'children'.