مَفْعُولٌ بِهِ is the person or thing upon whom or which the action falls.
In أَكَلَ إِبْرَاهِيْمُ التُّفَّاحَ — Ibrahim ate the apple:
- the action is eating;
- إِبْرَاهِيْمُ performed it, so he is the fa'il;
- the eating affected التُّفَّاحَ, so it is the maf'ul bihi.
Ask: “Whom or what did the action affect?” The answer often leads you to the maf'ul bihi.