مَفْعُولٌ لَهُ is a mansub noun that states the cause or reason for an action. It is also called مَفْعُولٌ لِأَجْلِهِ.
The word used for this reason is a verbal noun. It names an idea such as discipline, gratitude, fear or caution.
A useful question is: Why did the action happen? The verbal noun that answers this question may be the maf'ul lahu.