This is the 12192nd most frequent Polish word.
materacu
"Materacu" means "mattress" in English (genitive or locative singular form).
The word "materacu" here is the locative form, used after the preposition "na" to denote location.
Śpię na materacu, który jest bardzo wygodny.
I am sleeping on a mattress that is very comfortable.
In this sentence, the word "materacem" represents the instrumental case, highlighting its role in conjunction with the preposition "pod."
Znalazłem zagubiony telefon pod materacem.
I found the lost phone under the mattress.
Here, "materac" is in its basic nominative form as the object of the preposition related to its direct case usage.