You should look up the declensions of adjectives in Romanian. Basically, "bun"(good) is a four-form adjective. That means it has four different forms depending on the gender and number of the noun:

bun - masculine, singular

bună - feminine, singular

buni - masculine, plural

bune - feminine, plural

Since "friptură" is a feminine, singular noun, we use "bună".

Most adjectives have four forms, but there are such with 3, 2 or even 1.