This is the 17357th most frequent Polish word.
zapełniły
"Zapełniły" means "they filled" or "they have filled."
In this sentence, 'zapełnili' (filled) is used to indicate that people occupied the space in the hall completely, making it crowded for the event.
Ludzie zapełnili salę przed rozpoczęciem koncertu.
People filled the hall before the concert began.
Here, 'zapełniły' (filled themselves) is applied metaphorically to show that thoughts became full of particular memories or content.
Moje myśli zapełniły się wspomnieniami z dzieciństwa.
My thoughts were filled with childhood memories.
In this example, 'zapełniły' (were filled) describes how the shelves became loaded with new books due to the recent shipment.