"고양이가 빵을 먹습니다."
Translation:The cat eats bread.
의 is a possessive particle as in 's in direct translation to English. While 을 is an object marking particle, so 을 was used in front of 빵 because probably 빵 (bread) was the object in the sentnce. The writer didn't mean to say 'bread's' so tge possessive particle 의 wasn't used but rather 을 was used to indicate that 빵 was the object in that sentence.
I hoped this helped