This is the 4911th most frequent Polish word.
piasku
"Piasku" means "sand" in English (genitive form of "piasek").
In this sentence, 'piasku' is the locative form of the noun 'piasek', used to describe the location where the child is playing.
Dziecko bawi się w piasku.
The child is playing in the sand.
In this sentence, 'piasku' is the genitive form of the noun 'piasek', used to indicate the material that was poured.
Nasypano tutaj dużo piasku.
A lot of sand was poured here.
Here, 'piasku' is also in the genitive case, used for expressing possession or reference to the sand in focus.