This is the 4635th most frequent Polish word.
mleka
"Mleka" means "milk" (genitive singular or nominative plural) in Polish.
'mleka' is the genitive singular form of the noun 'mleko,' used here to indicate a partitive or non-specific quantity.
Dodałem do kawy trochę mleka.
I added some milk to the coffee.
'mleka' is again in the genitive singular form, used in negation with 'nie ma' to indicate absence.
Nie ma już w lodówce mleka.
There is no milk left in the fridge.
Here, 'mleka' is in the genitive singular due to the association with a measurement (litr).