The prefrontal cortex (located at the front, or in the anterior part, of the frontal lobes) is thought to be involved in concentration, although the brainstem is also involved in focusing. However, when something or someone is distracted and loses concentration, it's thought that another part of the brain takes over, specifically an area called the parietal cortex (or parietal lobe). The parietal cortex is located behind (posterior to) the frontal lobe.

Here's a news article that came out on a related scientific study in 2007: article