This is the 2060th most frequent Polish word.
matką
"Matką" means "mother" in English (instrumental case).
The word 'matką' is the instrumental singular form of 'matka' (mother), used to indicate a relationship or accompaniment in the instrumental case.
Spotkałem się dzisiaj z moją matką.
I met with my mother today.
Here, the word 'matką' is in instrumental singular form, used after a preposition 'o' indicating the topic of concern with a restricted grammatical structure.
Martwi się o swoją matką jak zawsze.
She worries about her mother as always.
In this instance, 'matką' is instrumental singular form used following the preposition 'o', fulfilling the case requirement.