This is the 17393rd most frequent Polish word.
zapłacili
"Zapłacili" means "they paid" in English.
In this sentence, 'zapłacili' indicates that the action of paying has been completed in the past tense, specifically referring to 'they' (third person plural).
Zapłacili za zakupy kartą kredytową.
They paid for the shopping with a credit card.
Here, 'zapłacili' is used similarly but directed towards compensating someone for their service.
Zapłacili mu za wykonanie pracy.
They paid him for completing the work.
This usage of 'zapłacili' conveys that the action took place and can be finalized (perfective aspect).