Polish adjectives and nouns change. This is called declension. Nouns change with cases (there are 7) and number (singular or plural), Nouns also have grammatical gender.
Adjectives have to match nouns- they change with gender, case and number.
If the noun is a subject of the sentence it is in case called nominative=mianownik. czarne skarpety
The object of verb mieć is in case called accusative. As it happens for "czarne skarpety" , accusative = nominative. So "Masz czarne skarpety"
If the verb is negated, instead of accusative you need case called genitive=dopełniacz,
Nie masz czarnych skarpet.