This is the 15740th most frequent Polish word.
szklanek
"Szklanek" is the genitive plural form of "szklanka," meaning "glasses" (as in drinking glasses).
Here, 'szklanek' is used in the locative plural form, indicating the location to where the juice was poured.
Janek nalał soku do szklanek.
Janek poured juice into the glasses.
Here, 'szklanki' is used in the nominative plural form to refer to 'glasses' as objects.
Na stole znajdują się trzy szklanki.
There are three glasses on the table.
In this sentence, 'szklanek' is in the genitive plural form, used after 'potrzebuję' which requires the genitive case.