The question is asking why people have trouble concentrating sometimes. Why do their minds wander?
What causes a person to be easily distracted from a task or focus he/she has chosen and believes is important?
Does that help?
Basically it is asking you to think about the parts of the brain and their function. So it's asking what part of the brain if damaged would result in a personal not being being able to concentrate and retrieve information despite continuously sending signals to.
If it is merely an aspect of attention, here is an article that might help:
In terms of brain areas, the hippocampus involves converting material in short-term memory (STM, 8-10 seconds without rehearsal) into long-term memory (LTM). If it doesn't get into LTM, there is nothing there to be retrieved.
Once information does get into LTM, the retrieval can vary depending on the type of memory (e.g., episodic, semantic, procedural) and the method used to measure retention (e.g., recall, recognition, relearning).
It gets more complex from there. Previous activities can help/hurt memories of newer material (e.g., positive/negative transfer), and newer material can help/hinder your memory of previously learned material (e.g., retroactive facilitation/inhibition).
However, the processes mentioned in the last two paragraphs have not been linked to specific brain areas.
I hope this helps a little more.