Distractibility refers to the difficulty or inability to focus on a task in the face of interruptions or other stimuli. This is one of the measures that is used in the identification of Attention-deficit disorder (ADD) or Attention-deficit hyperactivity disorder (ADHD). Sufferers of these disorders find it extremely difficult or impossible to focus on a task to completion- instead they will begin focusing on other tasks or stimuli around them.
An example would be a student who instead of taking an exam would constantly be distracted by normal background noises (a ticking clock or the sound of a student writing) or visual stimuli. In a school setting distractibility can be be managed by providing a quiet classroom environment with distractions being kept to a minimum. It should be noted that distractibility happens to everyone - how many times have you caught yourself daydreaming during a less than exciting subject in class? It is only when distractibility is so intrusive that it interferes with everyday functions that it can be a problem.