This is the 630th most frequent Polish word.
małych
"Małych" means "small" or "little" in the genitive or locative plural, or in the genitive singular feminine form.
The word 'małych' modifies the plural noun 'kotów' (cats) in the genitive case, used here due to the requirement of the word 'zwłaszcza' (especially).
Lubię zwierzęta, zwłaszcza małych kotów.
I like animals, especially small cats.
The word 'małych' modifies the plural noun 'domów' (houses) in the genitive case, as needed by the preposition 'z' (from).
Na podwórku bawią się dzieci z małych domów.
Children from small houses are playing in the yard.
The word 'małych' modifies the plural noun 'instrumentach' (instruments), which is in the locative case here, dictated by the preposition 'na' (on) in this context.